Huge reduction in solid fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
(I know that huge can be written as colossal)
'reduction in solid fuel' is the wordplay.
'reduction' becomes 'loss' (I've seen this in other clues).
'in' indicates putting letters inside.
'solid fuel' becomes 'coal' (I've seen this before).
'loss' going inside 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Great" , "So locals may be huge" , "Mammoth" , "Stupendous" , "Gargantuan" .)