Bubble in black thick liquid (4)
I believe the answer is:
boil
'bubble' is the definition.
(boiling water will bubble)
'black thick liquid' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'thick liquid' becomes 'oil' (I have seen 'Slippery liquid ' mean 'oil' so perhaps 'liquid' could also mean 'oil'. I am not sure about the 'thick' bit.).
'b'+'oil'='BOIL'
'in' is the link.
Can you help me to learn more?
(Other definitions for boil that I've seen before include "Bubble up - inflamed swelling" , "Cook in hot water" , "Cook, swelling" , "Painful sore" , "7 in water" .)