Grown up friend taking in old city (6)
I believe the answer is:
mature
'grown up' is the definition.
(I've seen this in another clue)
'friend taking in old city' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'taking in' means one lot of letters goes inside another.
'old city' becomes 'ur' (ancient city state).
'mate' enclosing 'ur' is 'MATURE'.
(Other definitions for mature that I've seen before include "Ripe or fully grown" , "Grown-up" , "Age" , "Grow older" , "due to be paid" .)