My first source of aromatic resin (5)
I believe the answer is:
Here is my best explanation:
'aromatic resin' is the definition.
(I've seen this before)
'my first source' is the wordplay.
I cannot really understand how this works, but
'my' is within the answer.
This may be the basis of clue (or it may be nonsense).
'of' is the link.
(Other definitions for myrrh that I've seen before include "birthday gift", "Frankincense and - -", "One gift of the Magi", "Wise Men's gift", "An essential oil", "Bitter tree gum, used in perfume and incense", "Gift from the Magi", "Gold, frankincense and - -", "fragrant substance" and "One of the first ever Christmas presents".)