Difficult thug upset about nothing (5)
I believe the answer is:
tough
'difficult' is the definition.
(similar in meaning)
'thug upset about nothing' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'thug' anagrammed gives 'tugh'.
'tugh' enclosing 'o' is 'TOUGH'.
(Other definitions for tough that I've seen before include "Unlucky" , "Durable - difficult" , "Difficult to cut and chew" , "Hardy and strong" , "Resilient; thug" .)