Defensive ditches (5)
I believe the answer is:
moats
'defensive ditches' 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 "Water-filled ditches as fortifications" , "Trenches around castles" , "Defensive water-filled ditches" , "Ditches around castles" , "Water defences" .)