For some fighters clinching is a way to get their opponent frustrated (out of their gameplan) to the point of getting careless/wreckless. Â Also if you are ever in trouble it's never a bad idea to clinch. Â Just like any other part of boxing, leverage and angles are a big part of being able to control a clinch. Â Never let someone just lean on you, that will take energy from you while he's resting, if he knows what he's doing. I'm not sure if that's the case here, or you are just being held (there is a difference).
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 Try a half of a step to one side or the other while getting your glove to the outside of his arm (right glove to his left elbow or vice versa). A quick push behind the elbow and this could be a way out.  No matter what you need to win the battle of position and balance.  Pushing him straight back with both of your gloves to both of his shoulders is an option, but only if his feet are squared at you, he will then be of balance and easy to push.  If his feet are set properly this takes to much energy to do repeatedly, as his base is solid and he will not move much with the shove.
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 Work your angles (stay on balance while trying to get him out of position),  position your gloves,  and never get frustrated to the point that you try to just muscle your way out or try to throw bomb after bomb. Remember to breathe properly and try to not tense up.
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 Good luck