Bar left in sack (4)
I believe the answer is:
axle
'bar' is the definition.
(I've seen this before)
'left in sack' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'sack' becomes 'axe' (both can mean to dismiss or cancel employment).
'l' going into 'axe' is 'AXLE'.
(Other definitions for axle that I've seen before include "Metal pin on which wheel revolves" , "It's pivotal" , "Wheel rotates around it" , "Shaft used for rotation" , "Bar on which wheel revolves" .)