Muhammad --, boxer (3)
I believe the answer is:
ali
This clue is a double definition.
'muhammad' is the first definition.
(I've seen this in another clue)
'boxer' is the second definition.
(Muhammad Ali)
(Other definitions for ali that I've seen before include "Muhammad ... (former fighter)" , "Muslim name" , "Old fighter" , "Champion of ring" , "Mohammad . . ., . . . Hewson" .)