Name adopted when following boxer (5)
I believe the answer is:
alias
'name adopted' is the definition.
'when following boxer' is the wordplay.
'when' becomes 'as'.
'following' means one lot of letters go next to another.
'boxer' becomes 'ali' (Muhammad Ali).
'as' put after 'ali' is 'ALIAS'.
(Other definitions for alias that I've seen before include "False or assumed name" , "it may be assumed" , "Otherwise - an assumed name" , "stage name, perhaps" , "another name" .)