Greek character in City blockbuster (4)
I believe the answer is:
epic
'blockbuster' is the definition.
(epic film)
'greek character in city' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'in' is an insertion indicator.
'city' becomes 'EC' (the City of London postcode).
'pi' placed within 'ec' is 'EPIC'.
(Other definitions for epic that I've seen before include "Vast in scale" , "Long poem/story" , "A long poem or book with adventurous theme" , "Guardian (body, beast)" , "Heroic; saga" .)