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 before).
'in' means one lot of letters goes inside another.
'fuel production' becomes 'coal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'loss' inserted 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" .)