Defensive ditch (4)
I believe the answer is:
moat
'defensive ditch' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for moat that I've seen before include "A deep water-filled ditch around a castle" , "Defensive water" , "Ditch dug to protect a castle" , "Trench around fortifications" , "defensive construction" .)