Iodine source (4)
I believe the answer is:
kelp
'iodine source' 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 "Algae" , "Tough brown seaweed" , "Large brown seaweed used for manure" , "Large seaweeds with leathery fronds" , "Large brown leathery seaweed" .)