Huge depletion in fossil fuel (8)
I believe the answer is:
colossal
'huge' is the definition.
('colossal' can be a synonym of 'huge')
'depletion in fossil fuel' is the wordplay.
'depletion' becomes 'loss'.
'in' means one lot of letters goes inside another.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'loss' placed within 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Stupendous" , "Massive" , "Huge, immense" , "Gargantuan" , "Mammoth" .)