Bird in castle (4)
I believe the answer is:
rook
This clue is a double definition.
'bird' is the first definition.
(rook is a kind of bird)
'castle' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for rook that I've seen before include "A bird; overcharge" , "Rip off - bird" , "Defraud -- crow" , "Man in game" , "man on board" .)