Sack not quite dry, a horror of mine (8)
I believe the answer is:
firedamp
'a horror of mine' is the definition.
(mining hazard)
'sack not quite dry' is the wordplay.
'sack' becomes 'fire' (both can mean to dismiss from a job).
'not quite dry' becomes 'damp' (I've seen this in another clue).
'fire'+'damp'='FIREDAMP'
(Other definitions for firedamp that I've seen before include "Methane in coalmines" , "dangerously explosive" , "a particular horror of mine" , "Explosive gas in mines" , "Combustible mixture" .)