Meaner pugilist using wrong side of fist (7)
I believe the answer is:
tighter
'meaner' is the definition.
('tight' can be a synonym of 'mean')
I don't understand the rest of the clue.
(Other definitions for tighter that I've seen before include "More close-fitting" , "more mean" , "More constricted" , "Less loose" , "more closely" .)