Bubble in black painting (4)
I believe the answer is:
boil
'bubble' is the definition.
(boiling water will bubble)
'black painting' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'painting' becomes 'oil' (oil painting is a kind of painting).
'b'+'oil'='BOIL'
'in' acts as a link.
(Other definitions for boil that I've seen before include "Abscess" , "Painful sore" , "become inflamed" , "7 in water" , "Eruption of the skin" .)