Clean, flash young boxer? (3,2)
I believe the answer is:
mop up
'clean' is the definition.
(to mop something up is to clean it)
'flash young boxer?' is the wordplay.
'flash' becomes 'mo' (both can mean a very short period of time).
'young boxer?' becomes 'pup' (boxer is a type of dog).
'mo'+'pup'='MOP-UP'
(Other definitions for mop up that I've seen before include "Clear away, dispose of" , "Swab" , "Finish a task, clear away remnants" , "take care of" , "Finish off a task, clean away remnants" .)