Something seen in boxing joint?
I believe the answer is:
uppercut
'something seen' is the definition.
I can't tell whether this defines the answer.
'boxing joint?' is the wordplay.
'boxing' becomes 'upper' (I can't explain this - if you can you should give a lot more credence to this answer).
'joint?' becomes 'cut' (joint is a kind of cut).
'upper'+'cut'='UPPERCUT'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for uppercut that I've seen before include "Blow from fist to chin" , "Swinging blow from below aimed at an opponent's chin" , "A vertical punch to chin delivered by boxer" , "Boxer's blow" , "Punch to opponent's chin from below" .)