Two bitters, one sent back in major face-off (4,3)
I believe the answer is:
cold war
'major face-off' is the definition.
'two bitters one sent back' is the wordplay.
'two bitters' means two substitutions for 'bitter'.
'bitter' becomes 'cold' ('cold' can be similar in meaning to 'bitter').
'bitter' becomes 'raw'.
'one sent back' shows that the letters should be reversed in order.
'raw' backwards is 'war'.
'cold'+'war'='COLD WAR'
'in' acts as a link.
(Other definitions for cold war that I've seen before include "Period of East-West political hostility" , "Intense hostility between nations without actually fighting" , "Chilly hostility between nations without actual fighting" , "State of hostility (rather than hostilities?)" , "Conflict without engagement" .)