Seriously harm leader of mob on purpose (4)
I believe the answer is:
maim
'seriously harm' is the definition.
(I've seen this in another clue)
'leader of mob on purpose' is the wordplay.
'leader of' indicates taking the first letters.
'on' says to put letters next to each other.
'purpose' becomes 'aim' (synonyms).
The initial letter of 'mob' is 'm'.
'm'+'aim'='MAIM'
(Other definitions for maim that I've seen before include "Injure seriously, leaving permanent damage" , "Injure (a person), causing permanent disability" , "Wound, injure, harm" , "Injure or harm severely" , "Wound or injure, disable" .)