Film can stored in Pennsylvania area (6)
I believe the answer is:
patina
'film' is the definition.
(both can mean a layer)
'can stored in pennsylvania area' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'stored in' means one lot of letters goes inside another.
'pennsylvania' becomes 'PA'.
'area' becomes 'a' (maths abbreviation).
'pa'+'a'='paa'
'tin' put inside 'paa' is 'PATINA'.
(Other definitions for patina that I've seen before include "Brownish surface film on old bronze" , "A surface gloss produced by age" , "Fine layer on surface (from oxidisation or handling)" , "well-worn surface" , "Film on copper or bronze" .)