Unfasten the duck cage (4)
I believe the answer is:
open
'unfasten' is the definition.
(thesaurus)
'the duck cage' is the wordplay.
'the duck' becomes 'o'.
'cage' becomes 'pen' (I've seen this before).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "such a verdict?" , "With which to offer hearty welcome" , "'Frank, not secret (4)'" , "unsealed" , "Traditional room heater" .)