Rod left chopper outside (4)
I believe the answer is:
axle
'rod' is the definition.
(axle is a kind of rod)
'left chopper outside' is the wordplay.
'left' becomes 'l' (common abbreviation).
'chopper' becomes 'axe' (something which chops).
'outside' is an insertion indicator.
'l' inserted into 'axe' is 'AXLE'.
(Other definitions for axle that I've seen before include "wheel section" , "Spindle between wheels" , "What supports rotor" , "Shaft used for rotation" , "Rod of wheelbase" .)