Protective ring (4)
I believe the answer is:
moat
'protective ring' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moat that I've seen before include "Water crossed via a drawbridge" , "Protective ditch" , "Ditch used as a defence" , "Defensive water-filled ditch" , "defensive construction" .)