Partner’s argument against — so right! (7)
I believe the answer is:
consort
'partner's' is the definition.
(synonyms)
'argument against so right' is the wordplay.
'argument against' becomes 'con' (I've seen this in another clue).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'con'+'so'+'rt'='CONSORT'
(Other definitions for consort that I've seen before include "Hang out with" , "Companion; ship sailing with another" , "A monarch's spouse" , "Spouse, partner" , "Husband or wife of a monarch" .)