Handle blow, retreating (4)
I believe the answer is:
knob
'handle' is the definition.
(knob is a kind of handle)
'blow retreating' is the wordplay.
'blow' becomes 'bonk' (both can mean a hit).
'retreating' says the letters should be written in reverse.
'bonk' written backwards gives 'KNOB'.
(Other definitions for knob that I've seen before include "Round door-handle" , "Round door handle or lump of butter" , "Rounded projection" , "Hump" , "Little group of wildfowl" .)