Defensive trenches (5)
I believe the answer is:
moats
'defensive trenches' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moats that I've seen before include "Ditches around castles" , "Water defences" , "Fortifications filled with water" , "Water-filled ditches as fortifications" , "Castle ditches" .)