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