New Conservative in assault offensive (6)
I believe the answer is:
rancid
'offensive' is the definition.
(thesaurus)
'new conservative in assault' is the wordplay.
'new' becomes 'n'.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'in' indicates putting letters inside.
'assault' becomes 'raid' (I've seen this in other clues).
'n'+'c'='nc'
'nc' going within 'raid' is 'RANCID'.
(Other definitions for rancid that I've seen before include "(Of fats) stale" , "Sour like stale grease 96)" , "Noxious" , "Putrid" , "Smelling bad like stale fat" .)