Massive deficit in fuel (8)
I believe the answer is:
colossal
'massive' is the definition.
(I've seen this before)
'deficit in fuel' is the wordplay.
'deficit' becomes 'loss' (I've seen this before).
'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 "Very big" , "Giant" , "Massive" , "Enormous, gigantic" , "So locals may be huge" .)