Extremely large depletion in fuel (8)
I believe the answer is:
colossal
'extremely large' is the definition.
(I've seen this before)
'depletion in fuel' is the wordplay.
'depletion' becomes 'loss'.
'in' means one lot of letters goes inside another.
'fuel' becomes 'coal' (coal is a kind of fuel).
'loss' placed into 'coal' is 'COLOSSAL'.
(Other definitions for colossal that I've seen before include "Huge, immense" , "Great" , "Enormous, gigantic" , "Mammoth" , "So locals may be huge" .)