Large-scale (4)
'large-scale' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for epic that I've seen before include "Long, heroic story" , "eg The Iliad" , "Tale of heroic deeds" , "long poem telling of heroic deeds" , "On a grand scale like long poem" .)
(Other definitions for mass that I've seen before include "Substance; a service" , "Body of matter" , "Matter; large amount" , "Total" , "Heap; religious service" .)