Cause of hay fever (6)
I believe the answer is:
pollen
'cause of hay fever' 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" , "Flower powder" , "Seed released by plant" , "Powdery substance given off by flowers" , "Fine powder given off by plants" .)