Cause of hay fever (6)
I believe the answer is:
pollen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cause of hay fever</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)