Marine backwater (3)
I believe the answer is:
ebb
'marine backwater' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for ebb that I've seen before include "Withdraw heartless" , "Type of tide" , "Abate" , "Tidal outflow" , "Go out like tide (3)" .)