Key film, a blockbuster (4)
I believe the answer is:
epic
'a blockbuster' is the definition.
(I've seen this before)
'key film' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'film' becomes 'pic' (short for picture).
'e'+'pic'='EPIC'
(Other definitions for epic that I've seen before include "A long poem or book with adventurous theme" , "imposing" , "Ben Hur, say" , "On a grand scale like long poem" , "Long narrative poem telling of a hero's deeds" .)