Black bird in castle (4)
I believe the answer is:
rook
I believe this clue is a double definition.
'black bird' is the first definition.
(I've seen this before)
'castle' is the second definition.
('rook' can be a synonym of 'castle')
'in' is the link.
(Other definitions for rook that I've seen before include "Defraud -- crow" , "16" , "flier" , "man for moving" , "Poignant, evocative" .)