Start of exciting film, a blockbuster (4)
I believe the answer is:
Here is my best explanation:
'blockbuster' is the definition.
(I've seen this before)
'start of exciting film' is the subsidiary indication.
'start of' says to take the initial letters.
'film' becomes 'pic' (I've seen this before**).
The initial letter of 'exciting' is 'e'.
'a' is the link.
(Other definitions for epic that I've seen before include "Impressively great", "Of heroic proportions", "Impressive scope", "blockbuster", "Long, heroic story", "Long account", "Ben Hur, say", "(Of) immense (proportions)", "Long story telling of a hero's deeds" and "Classic".)