Smart wife had picked up fighter (9)
I believe the answer is:
flyweight
'fighter' is the definition.
(I know that flyweight is a type of pugilist)
'smart wife had picked up' is the wordplay.
'smart' becomes 'fly' (fly can informally mean smart or well-dressed).
'wife' becomes 'w' (abbreviation).
'had' becomes 'ate' ('have' can mean 'eat').
'picked up' shows a homophone (sound like).
'ate' sounds like 'eight'.
'fly'+'w'+'eight'='FLYWEIGHT'
(Other definitions for flyweight that I've seen before include "Boxer weighing 51kg or less" , "Boxer in the lightest category" , "sporting division" , "Small, aggressive man" , "fighter" .)