A pint drunk before a film (6)
I believe the answer is:
patina
'film' is the definition.
(both can mean a layer)
'a pint drunk before a' is the wordplay.
'drunk' indicates anagramming the letters.
'before' means one lot of letters go next to another.
'a'+'pint'='apint'
'apint' is an anagram of 'patin'.
'patin'+'a'='PATINA'
(Other definitions for patina that I've seen before include "Surface film on ancient bronze" , "Fine layer on surface (from oxidisation or handling)" , "A surface gloss produced by age" , "Sheen produced by age and polishing" , "thin coating" .)