Keeps in with Bill the billionaire (5)
I believe the answer is:
baccy
'billionaire' is the definition.
(I know this)
'keeps in with bill' is the wordplay.
'keeps in' indicates putting letters inside.
'with' becomes 'by' (both can mean close together).
'bill' becomes 'acc'.
'by' placed around 'acc' is 'BACCY'.
'the' is the link.
(Other definitions for baccy that I've seen before include "Twist" , "Smoker's leaves (in short)" , "weed" .)