A friend finds ancient city unprofessional (7)
I believe the answer is:
amateur
'unprofessional' is the definition.
(synonyms)
'a friend finds ancient city' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'finds' means one lot of letters go next to another.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'a'+'mate'+'ur'='AMATEUR'
(Other definitions for amateur that I've seen before include "lacking professional expertise" , "lacking professionalism" , "hobbyist" , "11 [UNTRAINED]" , "I'm no expert!" .)