Duck stopping stops ducks (6)
I believe the answer is:
stoops
'ducks' is the definition.
(to stoop is to duck down)
'duck stopping stops' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'o' inserted inside 'stops' is 'STOOPS'.
(Other definitions for stoops that I've seen before include "Descends steeply (of a bird of prey)" , "Bends over" , "Goldsmith play 'She ...... to Conquer'" , "Condescends" .)