Anything but a terrible thug (5)
I believe the answer is:
aught
'anything' is the definition.
(thesaurus)
'a terrible thug' is the wordplay.
'terrible' is an anagram indicator.
'thug' with letters rearranged gives 'ught'.
'a'+'ught'='AUGHT'
'but' is the link.
(Other definitions for aught that I've seen before include "Nothing" , "Anything at all (archaic)" , "Anything (Shakespearean)" , "Emphatically anything" .)