Green supporter rejected importing iodine (4)
I believe the answer is:
naif
'green' is the definition.
(similar in meaning)
'supporter rejected importing iodine' is the wordplay.
'supporter' becomes 'fan' (fan is a kind of supporter).
'rejected' says the letters should be written in reverse.
'importing' is an insertion indicator.
'iodine' becomes 'I' (chemical symbol).
'fan' written backwards gives 'naf'.
'naf' going around 'i' is 'NAIF'.
(Other definitions for naif that I've seen before include "Green" , "Inexperienced" , "Unworldly" , "unaffected simplicity" , "somewhat ingenuous" .)