Underwater forest (4)
I believe the answer is:
kelp
'underwater forest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for kelp that I've seen before include "Tough brown seaweed" , "Large seaweeds with leathery fronds" , "Type of finch" , "Large brown leathery seaweed" , "Brown seaweed with fluted leathery fronds" .)