Main plant source of iodine (7)
I believe the answer is:
seaweed
This clue could be a double definition.
'main plant' is the first definition.
(I've seen this before)
'source of iodine' is the second definition.
'seaweed' can be an answer for 'source' (I have seen 'Iodine source ' mean 'seaweed' so perhaps 'source' could also mean 'seaweed'). I am not sure about the 'of iodine' bit.
Can you help me to learn more?
(Other definitions for seaweed that I've seen before include "Principal ingredient of laverbread" , "What may be cast up on shore" , "Wrack" , "Marine algae" , "Plant such as kelp" .)