Old city so backwardly modern (5)
I believe the answer is:
sodom
'old city' is the definition.
'sodom' can be an answer for 'city' (cities of the plain from Genesis). I am unsure of the 'old' bit.
'so backwardly modern' is the wordplay.
I cannot quite understand how this works, but
'so' is within the answer.
The remaining letters 'dom' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sodom that I've seen before include "Wicked depraved place" , "The piece of turf starts the wicked city" , "Twin town to Gomorrah" , "OT city destroyed for its wickedness" , "Place near Gomorrah" .)