Use taser on left-leaning eccentric (4)
'use taser' is the definition.
'left-leaning eccentric' is the wordplay.
'left-leaning' says the letters should be written backwards (letters written right to left).
'eccentric' becomes 'nuts' (I've seen this before).
'nuts' in reverse letter order is 'STUN'.
'on' acts as a link.
(Other definitions for stun that I've seen before include "Knock cold", "Stupefy - daze", "what a cracker may do?", "Daze, render semi-conscious", "Knock into a semi-conscious state".)