Gregarious crow (4)
I believe the answer is:
rook
'gregarious crow' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rook that I've seen before include "Chesspiece otherwise known as a castle" , "Species of crow" , "Swindle (colloq.); bird" , "Gregarious black bird" , "Chess piece also called castle" .)