Met to compose anthem (5)
I believe the answer is:
Here is my best explanation:
'anthem' is the definition.
'met to compose' is the wordplay.
'compose' indicates anagramming the letters.
'metto' with letters rearranged gives 'MOTET'.
(Other definitions for motet that I've seen before include "Piece of sacred music", "Short sacred choral work", "musical piece", "work by Tallis, for example", "Unaccompanied choral composition with sacred lyrics", "Totem (anag) ? anthem", "Church choral composition", "Certain one is morally superior", "Word; fixed period" and "Creating sacred piece".)