Former English West Country banker (3)
I believe the answer is:
exe
'west country banker' is the definition.
(River Exe, something with banks)
'former english' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'english' becomes 'E' (abbreviation as in OED).
'ex'+'e'='EXE'
(Other definitions for exe that I've seen before include "River of SW England" , "Current" , "English river sounds like a letter" , "River of southwest England" , "River flowing from Exmoor to Exmouth via Exeter" .)