Huge deficit in mine output (8)
I believe the answer is:
colossal
'huge' is the definition.
(I know that huge can be written as colossal)
'deficit in mine output' is the wordplay.
'deficit' becomes 'loss' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'mine output' becomes 'coal' (I've seen this in another clue).
'loss' placed inside 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Gargantuan" , "Enormous, gigantic" , "Giant" , "Extremely large" , "Huge, immense" .)