Watery defense (4)
I believe the answer is:
moat
'watery defense' 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 "Defensive water" , "Protective ditch" , "Castle surround" , "Trench round a castle" , "Ditch dug to protect a castle" .)