Anything that upsets a thug (5)
I believe the answer is:
aught
'anything' is the definition.
(thesaurus)
'upsets a thug' is the wordplay.
'upsets' indicates anagramming the letters.
'a'+'thug'='athug'
'athug' is an anagram of 'AUGHT'.
'that' acts as a link.
(Other definitions for aught that I've seen before include "Nothing" , "Anything at all (archaic)" , "Anything (Shakespearean)" , "Emphatically anything" .)