Huge financial deficit in fossil fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
(synonyms)
'financial deficit in fossil fuel' is the wordplay.
'financial deficit' becomes 'loss' (as in making a loss).
'in' indicates putting letters inside.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'loss' inserted into 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "So locals may be huge" , "Mammoth" , "Extremely large" , "Gargantuan" , "Very big" .)