Castle ditch (4)
I believe the answer is:
moat
'castle ditch' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for moat that I've seen before include "Trench round a castle" , "Ditch round castle" , "A deep water-filled ditch around a castle" , "Defensive water" , "Water-filled defensive trench" .)