On sale, oddly, old European city (4)
I believe the answer is:
oslo
'european city' is the definition.
(I've seen this before)
'on sale oddly old' is the wordplay.
'oddly' means one should take alternating letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'on'+'sale'='onsale'
The alternating letters of 'onsale' are 'osl'.
'osl'+'o'='OSLO'
(Other definitions for oslo that I've seen before include "Norwegian city, formerly Kristiania" , "City in Norway" , "where Ibsen lived under another name" , "European capital city (found in Czechoslovakia!)" , "Nordic capital" .)