Castle's trench (4)
I believe the answer is:
moat
'castle's trench' 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 "Trench around fortifications" , "Ditch round castle" , "Ditch dug to protect a castle" , "Defensive water-filled ditch" , "Last ditch defence?" .)