Mostly support quantity of gold for gambling game (8)
I believe the answer is:
baccarat
'gambling game' is the definition.
(I know that baccarat is a gambling game)
'mostly support quantity of gold' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'support' becomes 'back' (to back is to support or endorse).
'quantity of gold' becomes 'carat' (as in 24-carat gold etc).
'back' with its last letter taken away is 'bac'.
'bac'+'carat'='BACCARAT'
'for' acts as a link.
(Other definitions for baccarat that I've seen before include "French card game" , "Casino card-game" , "Gambling card-game" , "Card game in which players bet against the banker" , "Casino card game" .)