Around only a litre of fuel (4)

I believe the answer is:

coal

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'fuel' is the definition.
(I know that coal is a type of fossil fuel)

'around only a litre of' is the wordplay.
'around' indicates putting letters inside.
'only' becomes 'ca' (I can't justify this - if you can you should believe this answer much more).
'a litre' becomes 'L' (abbreviation).
'of' becomes 'o''.
'ca'+'l'='cal'
'cal' going around 'o' is 'COAL'.

Can you help me to learn more?

(Other definitions for coal that I've seen before include "Black mineral used for burning" , "Carbonaceous rock used as fuel" , "Mined fuel" , "slack" , "This consists mainly of carbon" .)

I've seen this clue in The Mirror.

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