Hay fever cause (6)
I believe the answer is:
pollen
'hay fever cause' 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 "Allergen from some plants" , "Fertilising grains" , "What sticks to insects" , "Powder from flowers" , "Fine powder produced by the anthers of seed-bearing plants" .)