Main unwanted growth is kelp (7)
I believe the answer is:
seaweed
'is kelp' is the definition.
(I've seen this in another clue)
'main unwanted growth' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'unwanted growth' becomes 'weed' (I've seen this in another clue).
'sea'+'weed'='SEAWEED'
(Other definitions for seaweed that I've seen before include "Principal ingredient of laverbread" , "Marine plant" , "Laver perhaps" , "Marine algae" , "Wrack" .)