That of a fuel can be high (6)
I believe the answer is:
octane
'be high' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'that of a fuel' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...
'can' is the link.
Can you help me to learn more?
(Other definitions for octane that I've seen before include "chemical" , "One cat (anag.)" , "A liquid hydrocarbon found in petroleum" , "Fuel component in McLaren vehicle" , "Hydrocarbon present in petrol" .)