Extensive killing of real thugs? (9)

I believe the answer is:

slaughter

I'm a little stuck... Click here to teach me more about this clue!

'extensive killing' is the definition.
'slaughter' can be an answer for 'killing' (slaughter is a kind of killing). I am not certain of the 'extensive' bit.

'of real thugs?' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'real'+'thugs'='realthugs'
'realthugs' anagrammed gives 'SLAUGHTER'.

Can you help me to learn more?

(Other definitions for slaughter that I've seen before include "best" , "Kill cruelly" , "Massacre" , "Glare thus to cause bloodthirsty killing" , "Murder" .)

I've seen this clue in The Irish Independent.

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