Boxer's cunning with rowers (9)
I believe the answer is:
flyweight
'boxer's' is the definition.
(flyweight is a kind of boxer)
'cunning with rowers' is the wordplay.
'cunning' becomes 'fly' (fly can mean skilful or sharp).
'with' becomes 'w' (abbreviation).
'rowers' becomes 'eight' (I've seen this before).
'fly'+'w'+'eight'='FLYWEIGHT'
(Other definitions for flyweight that I've seen before include "Boxer in the lightest category" , "Boxer weighing 51kg or less" , "Boxing category" , "sporting division" , "fighter" .)