Last of fuel? Burn coal (7)
I believe the answer is:
lignite
'coal' is the definition.
(lignite is a kind of coal)
'last of fuel? burn' is the wordplay.
'last of' suggests the final letters.
'burn' becomes 'ignite' (igniting is a kind of burning).
The last letter of 'fuel' is 'l'.
'l'+'ignite'='LIGNITE'
(Other definitions for lignite that I've seen before include "Brown coal" , "it'll burn" , "rock" , "Fuel" , "Soft brownish coal" .)