Old merchant ship making ancient city before day's end (6)
I believe the answer is:
argosy
'old merchant ship' is the definition.
(historical type of merchant ship)
'ancient city before day's end' is the wordplay.
'ancient city' becomes 'Argos' (ancient Greek city).
'before' is a charade indicator (letters next to each other).
'end' says to take the final letters.
The last letter of 'day' is 'y'.
'argos'+'y'='ARGOSY'
'making' is the link.
(Other definitions for argosy that I've seen before include "Merchant ship (poet.)" , "Great merchant ship" , "Large merchant ship (poetic)" , "Ancient merchant ship" , "One or more large merchant ships" .)