Disable a military leader on purpose (4)
I believe the answer is:
maim
'disable' is the definition.
(thesaurus)
'a military leader on purpose' is the wordplay.
'leader' says to take the initial letters.
'on' is a charade indicator (letters next to each other).
'purpose' becomes 'aim' ('aim' can be a synonym of 'purpose'**).
The initial letter of 'military' is 'm'.
'm'+'aim'='MAIM'
(Other definitions for maim that I've seen before include "Injure seriously, leaving permanent damage" , "Injure permanently" , "Severely damage" , "Wound and cripple" , "Hurt badly" .)