Huge decrease in fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
('colossal' can be a synonym of 'huge')
'decrease in fuel' is the wordplay.
'decrease' becomes 'loss' (loss is a kind of decrease).
'in' is an insertion indicator.
'fuel' becomes 'coal' (coal is a kind of fuel).
'loss' inserted into 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Huge, vast" , "Stupendous" , "Giant" , "Great" , "Extremely large" .)