Real thugs may make a killing (9)
I believe the answer is:
slaughter
'a killing' is the definition.
(I know that slaughter is a type of murder)
'real thugs may make' is the wordplay.
'may make' is an anagram indicator (I've seen this in other clues).
'real'+'thugs'='realthugs'
'realthugs' is an anagram of 'SLAUGHTER'.
(Other definitions for slaughter that I've seen before include "Butchery" , "Butcher" , "Kill cruelly" , "Kill (animals) for food" , "best" .)