Partner admits old city is developed as much as it can be (6)
I believe the answer is:
mature
'developed as much as it can be' is the definition.
I can't judge whether this defines the answer.
'partner admits old city' is the wordplay.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'admits' means one lot of letters goes inside another.
'old city' becomes 'ur' (ancient city state).
'mate' enclosing 'ur' is 'MATURE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for mature that I've seen before include "Full-grown - ripe" , "Ripen or grow up" , "reach the end of term" , "sensible" , "Old film star" .)