Start of exciting film, a blockbuster (4)
I believe the answer is:
epic
'a blockbuster' is the definition.
(I've seen this before)
'start of exciting film' is the wordplay.
'start of' suggests taking the first letters.
'film' becomes 'pic' (I've seen this in other clues**).
The first letter of 'exciting' is 'e'.
'e'+'pic'='EPIC'
(Other definitions for epic that I've seen before include "Long account" , "Vast in scale" , "Marathon" , "Long adventure story" , "Heroic; saga" .)