Ocean plant (7)
I believe the answer is:
seaweed
'ocean plant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seaweed that I've seen before include "Marine algae" , "what's been washed up" , "Bladderwrack, for example" , "Principal ingredient of laverbread" , "Marine plant" .)