At Ponders End, mail is sorted for the city (5)
I believe the answer is:
simla
'city' is the definition.
(I have seen 'Indian city' mean 'simla' so perhaps 'city' could also mean 'simla')
'at ponders end mail is sorted' is the wordplay.
'at' means one lot of letters go next to another.
'end' suggests the final letters.
'is sorted' indicates an anagram.
The final letter of 'ponders' is 's'.
'mail' with letters rearranged gives 'imla'.
's' next to 'imla' is 'SIMLA'.
'for the' is the link.
(Other definitions for simla that I've seen before include "Indian city" , "Hill station" , "Very distressed" , "Summer capital of the British Raj in India" , "state capital" .)