Flood-preventing barrier (5)
I believe the answer is:
levee
'flood-preventing barrier' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for levee that I've seen before include "Gathering" , "Embankment that should prevent flooding" , "protection for bank" , "Flood embankment" , "Retaining embankment" .)