Massive deficit in fuel production (8)
I believe the answer is:
colossal
'massive' is the definition.
(I've seen this before)
'deficit in fuel production' is the wordplay.
'deficit' becomes 'loss' (I've seen this in other clues).
'in' is an insertion indicator.
'fuel production' becomes 'coal' (I can't justify this - if you can you should believe this answer much more).
'loss' going within 'coal' is 'COLOSSAL'.
Can you help me to learn more?
(Other definitions for colossal that I've seen before include "Huge, vast" , "So locals may be huge" , "Extremely large" , "Mammoth" , "Enormous, gigantic" .)