Enormous deficit in fossil fuel (8)
I believe the answer is:
colossal
'enormous' is the definition.
(synonyms)
'deficit in fossil fuel' is the wordplay.
'deficit' becomes 'loss' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'loss' placed within 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Extremely large" , "Great" , "Massive" , "Giant" , "Enormous, gigantic" .)