Grand piano, one in church, returned (4)
I believe the answer is:
'grand' is the definition.
(I've seen this before)
'piano one in church returned' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (Church of England).
'returned' says the letters should be written backwards.
'ce' reversed gives 'ec'.
'pi' put into 'ec' is 'EPIC'.
(Other definitions for epic that I've seen before include "Surpassing the ordinary" , "Impressively great" , "Of heroic proportions" , "Long adventure story" , "Heroic narrative verse" .)