Barbaric bishop sits on struggling ultra (6)
I believe the answer is:
brutal
'barbaric' is the definition.
(thesaurus)
'bishop sits on struggling ultra' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'sits on' is a charade indicator (letters next to each other).
'struggling' indicates anagramming the letters (I've seen this in other clues).
'ultra' with letters rearranged gives 'rutal'.
'b'+'rutal'='BRUTAL'
(Other definitions for brutal that I've seen before include "Cruel, in animal fashion" , "Cruel and unfeeling like an animal" , "Very cruel, merciless" , "Very cruel and beastly" , "Harsh, merciless" .)