Sack of iron concealing iridium (4)
I believe the answer is:
fire
'sack' is the definition.
(both can mean to dismiss from a job)
'iron concealing iridium' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'concealing' is an insertion indicator.
'iridium' becomes 'Ir' (Ir is the chemical symbol for iridium).
'fe' enclosing 'ir' is 'FIRE'.
'of' acts as a link.
(Other definitions for fire that I've seen before include "cut" , "Inspire" , "Dismiss; bake in a kiln" , "See 5" , "Weaponry" .)