Real thugs involved in the killing (9)
I believe the answer is:
slaughter
'the killing' is the definition.
(slaughter is a kind of killing)
'real thugs involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'real'+'thugs'='realthugs'
'realthugs' is an anagram of 'SLAUGHTER'.
'in' acts as a link.
(Other definitions for slaughter that I've seen before include "Glare thus to cause bloodthirsty killing" , "Butchery" , "Widespread killing" , "Butcher" , "Massacre" .)