Zoo trench (4)
I believe the answer is:
moat
'zoo 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 ". . . part of defence" , "Ditch used as a defence" , "Defensive water-filled ditch" , "Trench around building" , "Last ditch defence?" .)