Drowned valley (3)
I believe the answer is:
ria
'drowned valley' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ria that I've seen before include "Institution for the promotion of art, in short" , "Long narrow inlet" , "Coastal inlet" , "Drowned river valley now an inlet of the sea" , "Submerged valley" .)