Nonprofessional colleague in a very old city (7)
I believe the answer is:
amateur
'nonprofessional' is the definition.
(similar in meaning)
'colleague in a very old city' is the wordplay.
'colleague' becomes 'mate' (mate can mean a colleague or friend).
'in' means one lot of letters goes inside another.
'very old city' becomes 'ur' (ancient Mesopotamian city).
'a'+'ur'='aur'
'mate' going within 'aur' is 'AMATEUR'.
(Other definitions for amateur that I've seen before include "he's not making any money" , "Receiving no money" , "Person who does something as a pastime rather than a profession" , "11 [UNTRAINED]" , "Dilettante: buff" .)