Brown seaweed (4)
I believe the answer is:
'brown seaweed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for kelp that I've seen before include "Large brown seaweed used for manure" , "Type of seaweed" , "Large brown leathery seaweed" , "Coarse brown seaweed" , "Brown seaweed with fluted leathery fronds" .)