Discretion of duke supporting game, dipping into cash (8)
I believe the answer is:
prudence
'discretion of duke' is the definition.
'prudence' can be an answer for 'discretion' (prudence is a kind of discretion). I'm not sure about the 'of duke' bit.
'supporting game dipping into cash' is the wordplay.
'supporting game' becomes 'rud' (I can't justify this - if you can you should believe this answer much more).
'dipping into' indicates putting letters inside.
'cash' becomes 'pence' (pennies).
'rud' placed within 'pence' is 'PRUDENCE'.
Can you help me to learn more?
(Other definitions for prudence that I've seen before include "Vigilance" , "Care and thought for the future" , "Discretion, caution" , "economy" , "Circumspection - girl's name" .)