Herald in old city at end of month (5)
I believe the answer is:
augur
'herald' is the definition.
(I've seen this in another clue)
'old city at end of month' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'at end of' says to put letters next to each other (some letters go at the end of others).
'month' becomes 'Aug' (abbreviation for August).
'ur' put after 'aug' is 'AUGUR'.
'in' acts as a link.
(Other definitions for augur that I've seen before include "Portend a good or/bad outcome" , "Prophesy" , "Reader of omens" , "Presage, portend" , "Portend, foretell" .)