Fuel was spread over grate, haphazardly (5,3)
I believe the answer is:
water gas
'fuel' is the definition.
(I know that water gas is a type of fuel)
'was spread over grate haphazardly' is the wordplay.
'spread over' means one lot of letters goes inside another.
'haphazardly' is an anagram indicator.
'grate' anagrammed gives 'aterg'.
'was' placed around 'aterg' is 'WATER-GAS'.
(Other definitions for water gas that I've seen before include "Synthetic fuel" , "source of fuel" , "Fuel made by passing steam over coke" .)