Enormous deficiency in fossil fuel (8)
I believe the answer is:
colossal
'enormous' is the definition.
(synonyms)
'deficiency in fossil fuel' is the wordplay.
'deficiency' becomes 'loss ' (I've seen this before).
'in' is an insertion indicator.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'loss' placed into 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Very big" , "So locals may be huge" , "Gargantuan" , "Enormous, gigantic" , "Great" .)