Clause for and against one, thus (7)
I believe the answer is:
proviso
'clause' is the definition.
(thesaurus)
'for and against one thus' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'against' becomes 'v' (abbreviation for 'versus'**).
'one' becomes 'i' (Roman numeral).
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'pro'+'v'+'i'+'so'='PROVISO'
(Other definitions for proviso that I've seen before include "Requirement" , "Stipulated condition" , "Condition attached to an agreement" , "Rider" , "Condition for VIP or so" .)