Disagree with work and sit (6)
I believe the answer is:
oppose
'disagree with' is the definition.
(thesaurus)
'work and sit' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'and' means one lot of letters go next to another.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'op'+'pose'='OPPOSE'
(Other definitions for oppose that I've seen before include "Bar" , "Resist, be hostile to" , "Resist, go against" , "Resist; play against" , "Take stand against, resist" .)