Green attends game between seasoned sides in private reserve (5,4)
I believe the answer is:
slush fund
'green attends game between seasoned sides' is the definition.
I can't tell whether this definition defines the answer.
'private reserve' is the wordplay.
'private' becomes 'slush' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reserve' becomes 'fund' (I have seen 'Reserve of money ' mean 'fund' so perhaps 'reserve' could also mean 'fund').
'slush'+'fund'='SLUSH FUND'
'in' is the link.
Can you help me to learn more?
(Other definitions for slush fund that I've seen before include "Sum of money to be used for bribery" , "Reserve of money used for bribery" , "Money to be used for financing corruption" , "Questionable political capital" .)