Small castle tower (6)
I believe the answer is:
turret
'small castle tower' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for turret that I've seen before include "Small tower on defensive wall - or tank" , "Part of tank" , "Little tower on a building" , "Feature of tank" , "Little (e.g. castle) tower" .)