Electronic image that's a blockbuster (4)
I believe the answer is:
epic
'a blockbuster' is the definition.
(I've seen this before)
'electronic image' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'image' becomes 'pic' (I've seen this before).
'e'+'pic'='EPIC'
'that's' acts as a link.
(Other definitions for epic that I've seen before include "Film or book on a grand scale" , "Confirm the truth of" , "Large in scale" , "of impressive proportions?" , "Large-scale" .)