Water-filled trench (4)
I believe the answer is:
moat
'water-filled trench' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moat that I've seen before include "Ditch used as a defence" , "Ditch dug to protect a castle" , "Water-filled defensive trench" , "Ditch round castle" , "Defensive water" .)