Wheel pin left in implement (4)
I believe the answer is:
axle
'wheel pin' is the definition.
(I've seen this in another clue)
'left in implement' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'implement' becomes 'axe' (axe is a kind of implement).
'l' inserted within 'axe' is 'AXLE'.
(Other definitions for axle that I've seen before include "Bar on which wheel revolves" , "Rod at centre of wheel" , "Rod supporting wheels" , "Rotating spindle" , "Wheel support" .)