Duck, possibly, served inside (4)
I believe the answer is:
bird
This clue is a double definition.
'duck possibly' is the first definition.
(duck is a kind of bird)
'served inside' is the second definition.
(bird can mean time served in prison)
(Other definitions for bird that I've seen before include "porridge" , "Flier - spell in prison" , "chat, perhaps?" , "Prison time (slang)" , "Nest builder?" .)