Common allergen (3)
'common allergen' is the definition.
(I've seen this in another clue)
This is the entire 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" .)
(Other definitions for nut that I've seen before include "Enthusiastic person" , "Hard shell of a locking device" , "eg Pecan" , "Fanatic (slang)" , "Hard fruit or informal lunatic" .)