Gift of the Magi (5)
'gift of the magi' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for myrrh that I've seen before include "One of the first ever Christmas presents" , "Fragrant resin burned as incense" , "Gift from the Wise Men - sweet cicely" , "One gift of the Magi" , "birthday gift" .)