Rye fungus (5)
I believe the answer is:
ergot
'rye fungus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ergot that I've seen before include "It's horny" , "Fungal disease of rye" , "drug supplier" , "Parasitical fungus" , "Fungal disease of grain" .)