The clinch is stand-up grappling, and it's where nearly every fight ends up if it lasts more than a few seconds. In serious self defense situations where two people are strongly clashing, if one doesn't take the other out with a strike they'll be grabbing each other very quickly. The clinch is often where fights are won or lost, even if it's only used momentarily. It allows one person to control another in order to apply powerful strikes, locks, breaks, chokes, and throws. Before a fight goes to the ground, unless one person falls to the ground on their own, it always passes through the clinch. Learning to strategically enter into the clinch to get a dominant position and apply optimal offensive and defense techniques is essential for both street and sport fighting.
Although it may not seem like it to those with little clinch experience, the clinch is also a safer place for smaller and weaker practitioners to go against larger and stronger opponents. This is due to the fact that it's nearly impossible to effectively punch in a tight clinch. And it's especially true when the smaller practitioner has better clinch skills than their opponent. A big man even if untrained can easily knock a small man or woman out with a hard strike in the face. But if he's not familiar with the clinch, it will be relatively easy to pummel him with powerful strikes and dirty tactics, to get to his back, throw him to the ground while simultaneously injuring him, or choke him out.
Most clinch fighting comes from combat sports such as western wrestling and judo. Striking is not allowed in these sports, so the positions and techniques most commonly taught and used there aren't necessarily the most optimal for self defense. Being able to knee, elbow, break joints, and use "dirty tactics" such as groin slaps, throat strikes, and eye gouges, creates the opportunity for positions and techniques not as common in clinch fighting for combat sports. With that said, the sport positions, transitions, and escapes are absolutely necessary. The difference in clinch fighting for self defense is in the additional positions and techniques. The training methods are still primarily the same.
Entering straight to the clinch can be an excellent self defense strategy, especially if your entry gives you immediate control of your opponent. Once you've gotten a superior position, you can decide how far you want to take it. If you've been seriously assaulted, using destructive techniques may be necessary. But if you've been attacked by a smaller person, a woman, a drunk family member, etc., you can choose to throw or lock your opponent without injuring them. Here are four entries I particularly like:
I find it best to begin clinch training without strikes or dirty tactics, and to add them in once a practitioner has developed skills in getting, maintaining, and escaping from each position. This way solid positional skills, which are fundamental in the clinch, can be developed more easily. Aside from learning the mechanics of each position and technique, all clinch training is sparring/wrestling for position, or wrestling for position + techniques.
At the isolation phase a single technique is drilled against a semi-cooperative opponent. For example, two practitioners begin in a low tie up and one drills the arm drag. At first, his partner allows him to do the technique, and then progressively resists it more and more. The same is done with every technique, including defense against particular techniques.
In full sparring integration, the game is played similar to in the isolation phase, only all techniques and counters are allowed.
There are useful goals that can be used as training exercises, such as getting your opponent's back against a wall, or getting your opponent to step over a marked line. These exercises can help practitioners learn to move their opponent effectively, in addition to applying techniques. For self defense, clinch training should be done in a variety of environments with a variety of clothing, where smashing your opponent into objects (safely!) is allowed.