Clear duck enclosure (4)
I believe the answer is:
open
'clear' is the definition.
('open' can be a synonym of 'clear')
'duck enclosure' is the wordplay.
'duck' becomes 'o'.
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Unrestricted; ready for business" , "Exposed to the air" , "Frank; not closed" , "Not shut or secret" , "ready to entertain new ideas" .)