Huge deficit in fossil fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
('colossal' can be a synonym of 'huge')
'deficit in fossil fuel' is the wordplay.
'deficit' becomes 'loss' (I've seen this before).
'in' means one lot of letters goes inside another.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'loss' going inside 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Mammoth" , "Stupendous" , "Massive" , "Enormous, gigantic" , "Jumbo" .)