Position sack and turn over light (3,4,2)
I believe the answer is:
set fire to
'light' is the definition.
(I've seen this before)
'position sack and turn over' is the wordplay.
'position' becomes 'set' (setting is a kind of positioning).
'sack' becomes 'fire' (both can mean to dismiss from a job).
'and' is a charade indicator (letters next to each other).
'turn over' becomes 'to' (t.o. can mean 'turn over' on a written document).
'set'+'fire'+'to'='SET FIRE TO'
(Other definitions for set fire to that I've seen before include "Ignite" , "torch" , "Light" .)