A companion at old city is unprofessional (7)
I believe the answer is:
amateur
'unprofessional' is the definition.
(synonyms)
'a companion at old city' is the wordplay.
'companion' becomes 'mate' (I've seen this before).
'at' says to put letters next to each other.
'old city' becomes 'ur' (ancient city state).
'a'+'mate'+'ur'='AMATEUR'
'is' acts as a link.
(Other definitions for amateur that I've seen before include "refusing payment" , "enthusiast" , "I'm no expert!" , "Not pro" , "Dabbler" .)