Setter's dog facing new attack (6)
I believe the answer is:
impugn
'attack' is the definition.
(thesaurus)
'setter's dog facing new' is the wordplay.
'setter's' becomes 'im' (I've seen this in other clues**).
'dog' becomes 'pug' (pug is a kind of dog).
'facing' says to put letters next to each other.
'new' becomes 'n'.
'im'+'pug'+'n'='IMPUGN'
(Other definitions for impugn that I've seen before include "Try to discredit, express doubts about honesty" , "Attack as false, challenge" , "Attack verbally" , "Dispute honesty of (someone's motive)" , "Express doubts about truth or honesty of, say motives" .)