Huge diminution in fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
('colossal' can be a synonym of 'huge')
'diminution in fuel' is the wordplay.
'diminution' becomes 'loss' (loss is a kind of diminution).
'in' indicates putting letters inside.
'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" , "So locals may be huge" , "Mammoth" , "Very big" , "Massive" .)