Politician getting alliance to occupy half of Serbia (7)
I believe the answer is:
senator
'politician' is the definition.
(I know that senator is a type of legislator)
'alliance to occupy half of serbia' is the wordplay.
'alliance' becomes 'nato' (NATO is an example).
'to occupy' indicates putting letters inside (inserted letters occupy or enter the word).
'half of' indicates taking half.
'serbia' cut in half is 'ser'.
'nato' inserted within 'ser' is 'SENATOR'.
'getting' is the link.
(Other definitions for senator that I've seen before include "Governor" , "Member of government" , "Member of the Second House" , "Politician involved in treason" , "One from uni's governing body" .)