Person who's investing gold in 17? (4)

I believe the answer is:

bull

'person who's investing' is the definition.
(bull is a type of investor)

I can't explain the rest of the clue.

(Other definitions for bull that I've seen before include "Papal document; farm animal" , "Tripe" , "Taurus in the Constellations" , "Papal proclamation" , "See 20 down" .)

I've seen this clue in The Guardian.

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