Big film very good in the City (4)
I believe the answer is:
'big film' is the definition.
'very good in the city' is the wordplay.
'very good' becomes 'pi' (old-fashioned term for 'pious').
'in the' means one lot of letters goes inside another.
'city' becomes 'EC' (the City of London postcode).
'pi' going within 'ec' is 'EPIC'.
(Other definitions for epic that I've seen before include "imposing" , "A long poem or book with adventurous theme" , "Of heroic proportions" , "Long poem/story" , "Tale of heroic deeds" .)