Great reduction in fuel (8)
I believe the answer is:
colossal
'great' is the definition.
(I know that great can be written as colossal)
'reduction in fuel' is the wordplay.
'reduction' becomes 'loss' (I've seen this in other clues).
'in' is an insertion indicator.
'fuel' becomes 'coal' (coal is a kind of fuel).
'loss' inserted within 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Jumbo" , "Stupendous" , "Enormous, gigantic" , "Huge, immense" , "Very big" .)