Sign of agreement is returned from old city (5)
I believe the answer is:
sidon
'old city' is the definition.
'sidon' can be an answer for 'city' (I have seen 'Ancient city' mean 'sidon' so perhaps 'city' could also mean 'sidon'). I'm not certain of the 'old' bit.
'sign of agreement is returned' is the wordplay.
'sign of agreement' becomes 'nod' (a nod can indicate agreement).
'returned' is a reversal indicator.
'nod'+'is'='nodis'
'nodis' written backwards gives 'SIDON'.
'from' acts as a link.
Can you help me to learn more?
(Other definitions for sidon that I've seen before include "Ancient Phoenician city" , "Chief city of ancient Phoenicia" , "It was allied to Tyre in Biblical times" .)