A water defence (4)
I believe the answer is:
moat
'a water defence' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for moat that I've seen before include "Water-filled ditch around stronghold" , "Castle surround" , "Water crossed via a drawbridge" , "Water-filled defensive trench" , "Trench round a castle" .)