Unidentified person on telephone, a West Country banker (3)
I believe the answer is:
exe
'a west country banker' is the definition.
(River Exe, something with banks)
'unidentified person on telephone' is the wordplay.
'unidentified person' becomes 'x' (X can represent an unknown person).
'on telephone' indicates a 'sounds like' (homophone) clue (as it might sound on the phone).
'x' sounds like 'EXE'.
(Other definitions for exe that I've seen before include "West Country river" , "River of SW England" , "River in SW UK" , "Topsham's river" , "Tiverton's river" .)