Small tower on a castle (6)
I believe the answer is:
turret
'small tower on a castle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for turret that I've seen before include "Small tower (on castle, etc)" , "Small corner tower" , "Armoured tower" , "Small tower, on tank or castle" , "Little (e.g. castle) tower" .)