Light sack carried in combat (3,4,2)
I believe the answer is:
set fire to
'light' is the definition.
(I've seen this before)
'sack carried in combat' is the wordplay.
'sack' becomes 'fire' (both can mean to dismiss from a job).
'carried in' indicates putting letters inside.
'combat' becomes 'set-to' (set-to is a kind of combat).
'fire' put into 'setto' is 'SET FIRE TO'.
(Other definitions for set fire to that I've seen before include "Ignite" , "torch" , "Light" .)