Huge deficit in fuel production (8)
I believe the answer is:
colossal
'huge' is the definition.
(I know that huge can be written as colossal)
'deficit in fuel production' is the wordplay.
'deficit' becomes 'loss' (I've seen this before).
'in' indicates putting letters inside.
'fuel production' becomes 'coal' (I can't justify this - if you can 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 "Very big" , "Giant" , "Jumbo" , "Great" , "So locals may be huge" .)