Crumpled banknote wrapped around a toxic food additive (5,4)
I believe the answer is:
tonka bean
'toxic food additive' is the definition.
(tonka beans can be toxic)
'crumpled banknote wrapped around a' is the wordplay.
'crumpled' indicates anagramming the letters (a crumbled or damaged version of the letters).
'wrapped around' means one lot of letters goes inside another.
'banknote' with letters rearranged gives 'tonkaben'.
'tonkaben' going around 'a' is 'TONKA-BEAN'.
(Other definitions for tonka bean that I've seen before include "Tree" , "flavouring agent" .)