Cruelty from low lives and leader of Mob (6)
I believe the answer is:
sadism
'cruelty' is the definition.
(I've seen this before)
'low lives and leader of mob' is the wordplay.
'low' becomes 'sad' (I've seen this before).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'and' means one lot of letters go next to another.
'leader of' says to take the initial letters.
The first letter of 'mob' is 'm'.
'sad'+'is'+'m'='SADISM'
'from' acts as a link.
(Other definitions for sadism that I've seen before include "Dim ass (anag)" , "Pleasure obtained by inflicting harm" , "The love of cruelty" , "Delight taken in cruelty" , "Sexual pleasure from hurting others" .)