Flue affected by petrol, say (4)
I believe the answer is:
fuel
'petrol say' is the definition.
(petrol is a kind of fuel)
'flue affected' is the wordplay.
'affected' indicates anagramming the letters.
'flue' with letters rearranged gives 'FUEL'.
'by' is the link.
(Other definitions for fuel that I've seen before include "Eg, coal or petrol" , "Combustibles" , "Gas, oil or petrol, for instance" , "Coal or gas?" , "coal etc" .)