UK banker first to take payment for letter (5)
I believe the answer is:
trent
'uk banker' is the definition.
(River Trent, something that 'banks')
'first to take payment for letter' is the wordplay.
'first to' says to take the initial letters.
'payment for letter' becomes 'rent' (payment made by someone letting a property).
The initial letter of 'take' is 't'.
't'+'rent'='TRENT'
(Other definitions for trent that I've seen before include "Eng. river; Council of -" , "Little Nell family; Eng. river" , "Burton's river" , "Nottingham's river" , "Midlands banker" .)