Impressive film to be seen online? (4)
I believe the answer is:
epic
'impressive' is the definition.
(epic can mean grand or impressive)
'film to be seen online?' is the wordplay.
I cannot quite see how this works, but
'film' could be 'pic' (short for picture) and 'pic' is found in the answer.
'online?' could be 'e' (as in e-mail or e-commerce) and 'e' is found within the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for epic that I've seen before include "Long narrative poem telling of a hero's deeds" , "Blown up" , "Heroic poem, film" , "Long poem - awesome!" , "Long adventure story" .)