Boxer considering that nickname (5)
I believe the answer is:
alias
'nickname' is the definition.
(both can mean an alternative name)
'boxer considering that' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'considering that' becomes 'as' (as can mean since).
'ali'+'as'='ALIAS'
(Other definitions for alias that I've seen before include "False or assumed name" , "Otherwise - an assumed name" , "Pseudonym" , "another name" , "Handle that isn't genuine" .)