Gas that makes sack wet (8)
I believe the answer is:
firedamp
'gas' is the definition.
(I know that firedamp is a type of gas)
'sack wet' is the wordplay.
'sack' becomes 'fire' (both can mean to dismiss from a job).
'wet' becomes 'damp' (similar in meaning).
'fire'+'damp'='FIREDAMP'
'that makes' acts as a link.
(Other definitions for firedamp that I've seen before include "Methane in coalmines" , "a particular horror of mine" , "Flammable gas formed in coal mines" , "danger to mines" , "dangerously explosive" .)