Ingredient of madeira is ... insidiously ... some old grapes (7)
I believe the answer is:
raisins
'some old grapes' is the definition.
(dried grapes)
'ingredient of madeira is insidiously' is the wordplay.
'ingredient of' indicates the answer is hidden within the clue.
'RAISINS' is hidden in the letters of 'madeira is insidiously'.
(Other definitions for raisins that I've seen before include "Fruit that's dried" , "Dried grapes used in cakes" , "Dried grapes, nice in cakes" .)