Common allergen (6)
I believe the answer is:
pollen
'common allergen' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pollen that I've seen before include "Frequent cause of allergies" , "Fine powder produced by the anthers of seed-bearing plants" , "Airborne 10 across" , "Powder from flowers" , "Fertilising grains" .)