Nothing unusual on thug (6)
I believe the answer is:
nought
'nothing' is the definition.
('nought' can be a synonym of 'zero')
'unusual on thug' is the wordplay.
'unusual' indicates an anagram.
'on'+'thug'='onthug'
'onthug' anagrammed gives 'NOUGHT'.
(Other definitions for nought that I've seen before include "The digit 0" , "Love" , "Zero" , "Poetically nothing" , "The figure 0" .)