Coating on a roll (4)
I believe the answer is:
film
I believe this clue is a double definition.
'coating' is the first definition.
(I have seen 'Thin coating' mean 'film' so perhaps 'coating' could also mean 'film')
'roll' is the second definition.
(thesaurus)
'on a' is the link.
(Other definitions for film that I've seen before include "Screen" , "Thin layer - seen on screens" , "Thin coating for movie." , "Motion picture" , "reels, perhaps" .)