Doe's mate (4)
(Other definitions for stag that I've seen before include "Animal - for men only" , "Investor" , "party given by Uncle Sam" , "Adult male deer, often with antlers" , "Antlered beast" .)
'doe's mate' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for buck that I've seen before include "(Of a trend) resist" , "some Australians try to make fast" , "Blood < money" , "young man" , "Jump - dollar - male deer" .)