Anything but a terrible thug (5)
I believe the answer is:
Here is my best explanation:
'anything' is the definition.
'a terrible thug' is the subsidiary indication.
'terrible' indicates an anagram.
'thug' anagrammed gives 'ught'.
'but' is the link.
(Other definitions for aught that I've seen before include "Nothing", "Anything at all (archaic)", "Anything (Shakespearean)" and "Emphatically anything".)