Ditch around a castle (4)
I believe the answer is:
moat
'ditch around a castle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moat that I've seen before include "defensive construction" , "Ditch used as a defence" , "A deep water-filled ditch around a castle" , "Last ditch defence?" , "Trench around fortifications" .)