Slur (not acceptable), curious, distorted, defamatory (10)
I believe the answer is:
scurrilous
'defamatory' is the definition.
(thesaurus)
'slur not acceptable curious distorted' is the wordplay.
'not' suggests deleting specific letters.
'acceptable' becomes 'u' (as in U and non-U speech).
'distorted' is an anagram indicator.
'curious' with 'u' taken out is 'crious'.
'slur'+'crious'='slurcrious'
'slurcrious' is an anagram of 'SCURRILOUS'.
(Other definitions for scurrilous that I've seen before include "Unjustifiably defamatory" , "Humorously insulting" , "Obscenely slanderous" , "Scandalously damaging" , "Grossly insulting" .)