Floodgate (6)
I believe the answer is:
sluice
'floodgate' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sluice that I've seen before include "Water flow control gate" , "really wet" , "Water drainage channel" , "Wash or rinse with a stream of water" , "Conduit for a rapid flow of water" .)