Unlock the duck coop (4)
I believe the answer is:
open
'unlock' is the definition.
(I know that unlock is a more specific form of the action open)
'duck coop' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'coop' becomes 'pen' (I've seen this before).
'o'+'pen'='OPEN'
'the' is the link.
(Other definitions for open that I've seen before include "Agape" , "Not enclosed" , "Candid - ready for business" , "Frank; unfold" , "Undecided" .)