Like a billionaires' club member, author reportedly has already joined? (7,2,2)
I believe the answer is:
rolling in it
'like a billionaires' club member' is the definition.
(rolling in it means very rich)
'author reportedly has already joined?' is the wordplay.
I cannot really understand how this works, but
'has already joined?' could be 'init' (in a group) and 'init' is present in the answer.
The remaining letters 'rolling' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Another definition for rolling in it that I've seen is " Rich".)