One game to plug venture capital (6)
I believe the answer is:
beirut
'capital' is the definition.
(capital of Lebanon)
'one game to plug venture' is the wordplay.
'one' becomes 'i' (Roman numeral).
'game' becomes 'RU' (rugby union).
'to plug' indicates putting letters inside (to plug can mean to block a gap within something).
'venture' becomes 'bet' (bet is a kind of venture).
'i'+'ru'='iru'
'iru' put into 'bet' is 'BEIRUT'.
(Other definitions for beirut that I've seen before include "Capital of 21 across" , "Capital city of Lebanon" , "Lebanese capital" , "Middle East capital" , "Lebanon's capital" .)