Narrow coastal inlet (3)
I believe the answer is:
ria
'narrow coastal inlet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ria that I've seen before include "Drowned coastal valley" , "Long narrow inlet" , "Flooded valley" , "inlet between hills" , "Irish academy for sciences and humanities, in short" .)