Brutally treated after backsliding, a politician offering insults (10)
I believe the answer is:
derogatory
'offering insults' is the definition.
(something derogatory is insulting)
'brutally treated after backsliding a politician' is the wordplay.
'brutally treated' becomes 'gored' (gore can mean to stab or pierce).
'after backsliding' says the letters should be written backwards.
'politician' becomes 'tory' (term for a Conservative Party politician).
'gored' back-to-front is 'derog'.
'derog'+'a'+'tory'='DEROGATORY'
(Other definitions for derogatory that I've seen before include "deliberately offensive" , "Insulting" , "Disparaging" , "Showing a disrespectful attitude" .)