Anthem a girl used to keep bearers of gifts (3,4)
I believe the answer is:
the magi
'bearers of gifts' is the definition.
(the Magi brought gifts to Jesus in the Bible)
'anthem a girl used to keep' is the wordplay.
'to keep' indicates a hidden word.
'THE MAGI' can be found hidden inside 'anthem a girl used'.
(Other definitions for the magi that I've seen before include "Eastern sages" , "Journeymen" .)