Ocean arm (5)
I believe the answer is:
inlet
'ocean arm' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inlet that I've seen before include "Arm off a larger body of water between headlands" , "Opening - passage" , "One's up the creek here" , "Arm of lake, river" , "A small creek or cove" .)