The loss in fuel is gigantic (8)
I believe the answer is:
colossal
'gigantic' is the definition.
(synonyms)
'the loss in fuel' is the wordplay.
'in' means one lot of letters goes inside another.
'fuel' becomes 'coal' (coal is a kind of fuel).
'loss' placed inside 'coal' is 'COLOSSAL'.
'is' is the link.
(Other definitions for colossal that I've seen before include "Massive" , "Huge, vast" , "Gargantuan" , "Jumbo" , "Great" .)