Material refers to toffee coating on Irish apple (5)
I believe the answer is:
tulle
'material' is the definition.
(tulle is a kind of material)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tulle that I've seen before include "that may be around one's body" , "Delicate material" , "Silk net fabric" , "Fine net fabric used for hat trimming, etc." , "Delicate netted cloth" .)