Arm of the sea (5)
I believe the answer is:
inlet
'arm of the sea' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for inlet that I've seen before include "Entry; arm of the sea" , "Strip of water extending from sea into land" , "bit of a lake here?" , "Opening through which liquid can flow" , "Small arm of the sea" .)