Privy to one French veto (2,2)
I believe the answer is:
in on
'privy to' is the definition.
(I've seen this in another clue)
'one french veto' is the wordplay.
'one' becomes 'i' (Roman numeral).
'french veto' becomes 'non' (French for 'no', a word which might veto something).
'i'+'non'='IN ON'
(Other definitions for in on that I've seen before include "Wise to" , "Sharing with" , "Privy to" , "Aware of - being part of" , "Participating in - knowing about" .)