It's water under the bridge! (4)
I believe the answer is:
moat
'it's water under the bridge' 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 "Water-filled ditch around stronghold" , "Castle's ditch" , "defensive construction" , "Ditch round castle" , "Ditch dug to protect a castle" .)