Sew up boxer's excellent clothing (8)
I believe the answer is:
finalise
'sew up' is the definition.
('sew up'->'finalize' is in my internal thesaurus and 'ize'->'ise')
'boxer's excellent clothing' is the wordplay.
'boxer's' becomes 'Ali's' (Muhammad Ali).
'excellent' becomes 'fine' (synonyms).
'clothing' is an insertion indicator (some letters cover up or clothe others).
'alis' inserted within 'fine' is 'FINALISE'.
(Other definitions for finalise that I've seen before include "Agree on definitive version of something" , "Put an end to, complete" , "Conclude" , "settle" , "Put the last touches to" .)