Trivial piece repulsed a politician (8)
I believe the answer is:
nugatory
'trivial' is the definition.
(thesaurus)
'piece repulsed a politician' is the wordplay.
'piece' becomes 'gun' (slang term for a gun).
'repulsed' shows that the letters should be reversed in order.
'politician' becomes 'tory' (term for a Conservative Party politician).
'gun' written backwards gives 'nug'.
'nug'+'a'+'tory'='NUGATORY'
(Other definitions for nugatory that I've seen before include "to little effect" , "useless" , "Insignificant or trivial" , "Worthless - futile" , "Any grout (anag) - trifling" .)