Water-filled ditch around a castle (4)
I believe the answer is:
moat
'water-filled ditch around a castle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for moat that I've seen before include "Castle's ditch" , "Water-filled trench around a castle" , "Water defence" , "Last ditch defence?" , "Ditch round castle" .)