Champion boxer with a second assumed name (5)
I believe the answer is:
alias
'assumed name' is the definition.
(synonyms)
'champion boxer with a second' is the wordplay.
'champion boxer' becomes 'Ali' (Muhammad Ali).
'with' says to put letters next to each other.
'second' becomes 's' (common abbreviation).
'ali'+'a'+'s'='ALIAS'
(Other definitions for alias that I've seen before include "another name" , "False identity" , "False or assumed name" , "Assumed identity" , "Otherwise - an assumed name" .)