Massive decrease in fossil fuel (8)
I believe the answer is:
colossal
'massive' is the definition.
(I've seen this before)
'decrease in fossil fuel' is the wordplay.
'decrease' becomes 'loss' (loss is a kind of decrease).
'in' is an insertion indicator.
'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 "Giant" , "Very big" , "Stupendous" , "Gargantuan" , "Great" .)