Banker gets cab finally entering rank (5)
I believe the answer is:
tiber
'banker' is the definition.
(I have seen 'Italian banker' mean 'tiber' so perhaps 'banker' could also mean 'tiber')
'cab finally entering rank' is the wordplay.
'finally' says to take the final letters.
'entering' means one lot of letters goes inside another.
'rank' becomes 'tier' (tier is a kind of rank).
The last letter of 'cab' is 'b'.
'b' put within 'tier' is 'TIBER'.
'gets' is the link.
(Other definitions for tiber that I've seen before include "The flower of Rome" , "Italian banker" , "River entering the Mediterranean at Ostia" , "divides Rome" , "Roman river" .)