Italian banker having billions in bank? (5)
I believe the answer is:
tiber
'italian banker' is the definition.
(I've seen this before)
'billions in bank?' is the wordplay.
'billions' becomes 'b' (abbreviation).
'in' is an insertion indicator.
'bank?' becomes 'tier' (both can mean a row of similar things).
'b' put within 'tier' is 'TIBER'.
'having' is the link.
(Other definitions for tiber that I've seen before include "Italian river with Ostia at its mouth" , "Roman river" , "River entering the Mediterranean at Ostia" , "River of Rome" , "divides Rome" .)