One cleaning firm needs a litre in fuel (8)
I believe the answer is:
charcoal
'fuel' is the definition.
(I've seen this before)
'one cleaning firm needs a litre' is the wordplay.
'one cleaning' becomes 'char' (I can't explain this - if you can you should believe this answer much more).
'firm' becomes 'co' (a firm is a company).
'needs' says to put letters next to each other.
'litre' becomes 'l' ('l' can be a synonym of 'litre').
'char'+'co'+'a'+'l'='CHARCOAL'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for charcoal that I've seen before include "Fuel - very dark grey" , "Form of carbon for drawing" , "What gets Barbie going" , "Fuel for barbecue cookers" , "Fuel made from burnt wood" .)