Cereal fungus (5)
I believe the answer is:
ergot
'cereal 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 "Fungal infection" , "drug supplier" , "Cereal disease" , "Fungal disease of grain" , "Could Roget have a fungal plant disease?" .)