Bubble in black fuel (4)
I believe the answer is:
boil
'bubble' is the definition.
(boiling water will bubble)
'black fuel' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'fuel' becomes 'oil' (oil is a type of fuel).
'b'+'oil'='BOIL'
'in' acts as a link.
(Other definitions for boil that I've seen before include "Skin swelling; cause to evaporate" , "become inflamed" , "Abscess" , "Bubble up - inflamed swelling" , "7 in water" .)