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 in other clues).
'in' means one lot of letters goes inside another.
'mine output' becomes 'coal' (I've seen this in another clue).
'loss' inserted within 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Gargantuan" , "Enormous, gigantic" , "Giant" , "Extremely large" , "Huge, immense" .)