Billionaire — important fixative in perfumery (4)

I believe the answer is:

musk

(Other definitions for musk that I've seen before include "Body odour" , "that smells strongly" , "Scented glandular secretion from a deer" , "Substance from deer used as basis of perfumes" , "Glandular secretion from a male deer, used in making perfume" .)

I've seen this clue in The Guardian.

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