Wipe the floor with flash young boxer? (3,2)
I believe the answer is:
mop up
'wipe the floor' is the definition.
(as in mopping a floor)
'with flash young boxer?' is the wordplay.
'with' is a charade indicator (letters next to each other).
'flash' becomes 'mo' (both can mean a very short period of time).
'young boxer?' becomes 'pup' (boxer is a type of dog).
'mo'+'pup' is 'MOP-UP'.
(Other definitions for mop up that I've seen before include "Complete final stage (of a task)" , "Clear away, dispose of" , "Kill (enemy stragglers)" , "polish off" , "Deal with spillage or defeated enemy" .)