Monumental (4)
'monumental' is the definition.
(epic can mean monumental or impressive)
This is the entire clue.
(Other definitions for epic that I've seen before include "Ben Hur, say" , "Large-scale" , "Film or book on a grand scale" , "Long literary work about great events" , "Nonsense, rubbish" .)
'monumental' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for big that I've seen before include "London bell" , "Large or substantial" , "oversized" , "Large; generous" , "Important" .)