Relation of a city to its auditors (5)

I believe the answer is:
niece
'relation' is the definition.
(niece is a kind of relation)
'city to its auditors' is the wordplay.
I cannot quite see how this works, but
'city' could be 'ec' (the City of London postcode) and 'ec' is found in the answer.
The remaining letters 'nie' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of a' is the link.
Can you help me to learn more?
(Other definitions for niece that I've seen before include "A sibling's child" , "A sibling's daughter" , "Daughter of a sibling" , "A younger female relative" , "Female family member" .)
