Ancient city on eastern river (3)
I believe the answer is:
ure
'river' is the definition.
(I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure')
'ancient city on eastern' is the wordplay.
'ancient city' becomes 'ur'.
'on' says to put letters next to each other.
'eastern' becomes 'e'.
'ur'+'e'='URE'
(Other definitions for ure that I've seen before include "Ripon's river" , "River at Ripon" , "Midge --, singer" , "River of N. Yorkshire" , "Wensleydale river" .)