Wager about one game in Mediterranean capital (6)
I believe the answer is:
beirut
'mediterranean capital' is the definition.
(capital of Lebanon, country on the Mediterranean)
'wager about one game' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'game' becomes 'RU' (rugby union).
'i'+'ru'='iru'
'bet' placed around 'iru' is 'BEIRUT'.
'in' acts as a link.
(Other definitions for beirut that I've seen before include "Capital of 21 across" , "Capital city of Lebanon" , "Middle East capital" , "Lebanese capital" , "Lebanon's capital" .)