Cut legs when retreating (4)
I believe the answer is:
snip
'cut' is the definition.
(snip is a kind of cut)
'legs when retreating' is the wordplay.
'legs' becomes 'pins' (pin is a kind of leg).
'when retreating' says the letters should be written backwards.
'pins' in reverse letter order is 'SNIP'.
(Other definitions for snip that I've seen before include "Bargain (informal)" , "Cut back" , "Cut - prune" , "Cut with scissors in small strokes" , "nick)" .)