Old union was really against tie-breaks (6)
I believe the answer is:
soviet
'old union' is the definition.
(I've seen this in another clue)
'really against tie-breaks' is the wordplay.
'really' becomes 'so' (both can mean 'extremely').
'against' becomes 'v' (abbreviation for 'versus').
'breaks' indicates anagramming the letters.
'tie' anagrammed gives 'iet'.
'so'+'v'+'iet'='SOVIET'
'was' is the link.
(Other definitions for soviet that I've seen before include "Old government council" , "Old Russian council" , "National council in former USSR" , "Like the USSR" , "Union of ... Socialist Republics" .)