Prepare to whistle for one's money (5)

I believe the answer is:

purse

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'money' is the definition.
(a purse contains money)

I don't understand the remainder of the clue.

(Other definitions for purse that I've seen before include "Money bag; pucker" , "Small pouch for carrying money, mostly by women" , "bag for loose change" , "Sum of money as a prize - in pouch?" , "Moneybag" .)

I've seen this clue in The Globe and Mail.

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