Rook (6)
I believe the answer is:
castle
'rook' is the definition.
(I know that rook can be written as castle)
This is the entire clue.
(Other definitions for castle that I've seen before include "Chess piece - stronghold" , "Fortress; rook" , "ancient building?" , "Fortified place on the chess board" , "Let sac be a chess piece" .)