The deficit in fuel is massive (8)
I believe the answer is:
colossal
'massive' is the definition.
(I've seen this before)
'the deficit in fuel' is the wordplay.
'the 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 within 'coal' is 'COLOSSAL'.
'is' is the link.
(Other definitions for colossal that I've seen before include "Mammoth" , "Jumbo" , "Stupendous" , "Gargantuan" , "So locals may be huge" .)