Perhaps Stallone's accepting old agents' bill, initially in a friendly manner (8)
I believe the answer is:
sociably
'perhaps stallone's' is the definition.
I can't judge whether this defines the answer.
'accepting old agents' bill initially in a friendly manner' is the wordplay.
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'agents'' becomes 'CIA' (Central Intelligence Agency).
'initially' suggests taking the first letters.
'in a friendly manner' becomes 'sly' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'bill' is 'b'.
'o'+'cia'+'b'='ociab'
'ociab' put inside 'sly' is 'SOCIABLY'.
Can you help me to learn more?
(Another definition for sociably that I've seen is " In a genial manner".)