Boxer went to ground briefly (3)
I believe the answer is:
ali
'boxer' is the definition.
(Muhammad Ali)
'went to ground briefly' is the wordplay.
'went to ground' becomes 'alit' ('alight' can mean to land or reach the ground).
'briefly' means to remove the last letter.
'alit' with its final letter taken away is 'ALI'.
(Other definitions for ali that I've seen before include "Person" , "A champ'" , "Mohammed . . . (boxer)" , "Son-in-law to Muhammad" , "Muhammad ..., famous boxer" .)