Capital one invested in new casino (7)
I believe the answer is:
nicosia
'capital' is the definition.
(capital of Cyprus)
'one invested in new casino' is the wordplay.
'one' becomes 'i' (Roman numeral).
'invested in' indicates putting letters inside.
'new' indicates an anagram.
'casino' is an anagram of 'nicosa'.
'i' inserted into 'nicosa' is 'NICOSIA'.
(Other definitions for nicosia that I've seen before include "Casino I visit in Cyprus" , "It's capital in Cyprus (7)" , "Casino I used in Cyprus" , "Mediterranean city" , "European Union's most southerly capital" .)