Allergen from some plants (6)
I believe the answer is:
pollen
'allergen from some plants' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pollen that I've seen before include "Cause of hay fever" , "Airborne 10 across" , "Powder from flowers" , "Seed released by plant" , "Allergen from some plants" .)