Iodine-rich seaweed (4)
I believe the answer is:
kelp
'iodine-rich seaweed' 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 "Type of finch" , "Large brown seaweed used for manure" , "Brown seaweed with fluted leathery fronds" , "Type of seaweed" , "Algae" .)