Huge deficit in mine output (8)
I believe the answer is:
colossal
'huge' is the definition.
(synonyms)
'deficit in mine output' is the wordplay.
'deficit' becomes 'loss' (I've seen this before).
'in' indicates putting letters inside.
'mine output' becomes 'coal' (I've seen this in another clue).
'loss' placed within 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Gargantuan" , "Enormous, gigantic" , "Giant" , "Extremely large" , "Huge, immense" .)