Cavalier treatment for oily tars? (8)
I believe the answer is:
royalist
'cavalier' is the definition.
(Cavaliers were royalists in the English Civil War)
'treatment for oily tars?' is the wordplay.
'treatment for' indicates anagramming the letters (I've seen this in other clues).
'oily'+'tars'='oilytars'
'oilytars' anagrammed gives 'ROYALIST'.
(Other definitions for royalist that I've seen before include "Civil War Cavalier" , "Solitary (anag.)" , "12 (anag)" , "Supporter of monarchy" , "Monarch's supporter" .)