Boxer Muhammad's breaking excellent clinch (8)
I believe the answer is:
finalise
'clinch' is the definition.
('clinch'->'finalize' is in my internal thesaurus and 'ize'->'ise')
'boxer muhammad's breaking excellent' is the wordplay.
'boxer muhammad's' becomes 'Ali's' (Muhammad Ali).
'breaking' means one lot of letters goes inside another.
'excellent' becomes 'fine' (synonyms).
'alis' inserted inside 'fine' is 'FINALISE'.
(Other definitions for finalise that I've seen before include "Put an end to, complete" , "settle" , "Agree on definitive version of something" , "Put the last touches to" , "Conclude" .)