Bubble in black paint (4)
I believe the answer is:
boil
'bubble' is the definition.
(boiling water will bubble)
'black paint' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'paint' becomes 'oil' (oil paint is a kind of paint).
'b'+'oil'='BOIL'
'in' is the link.
(Other definitions for boil that I've seen before include "Cook, swelling" , "Eruption of the skin" , "Painful sore" , "Cook in hot water" , "Sore with a hard core filled with pus" .)