Layer on diseased bean plant (7)
I believe the answer is:
henbane
'plant' is the definition.
(henbane is a kind of plant)
'layer on diseased bean' is the wordplay.
'layer' becomes 'hen' (I've seen this in another clue).
'on' is an insertion indicator.
'diseased' indicates anagramming the letters (I've seen this in other clues).
'bean' with letters rearranged gives 'bane'.
'hen' put into 'bane' is 'HENBANE'.
(Other definitions for henbane that I've seen before include "poison" , "Poisoner" , "Poisonous plant" .)