Disable a military leader on purpose (4)
I believe the answer is:
maim
'disable' is the definition.
(I've seen this before)
'a military leader on purpose' is the wordplay.
'leader' suggests taking the first letters.
'on' means one lot of letters go next to another.
'purpose' becomes 'aim' (synonyms).
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" .)