Black, black liquid swelling (4)

I believe the answer is:

boil

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'swelling' is the definition.
(thesaurus)

'black black liquid' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'black liquid' becomes 'oil' (I have seen 'Slippery liquid ' mean 'oil' so perhaps 'liquid' could also mean 'oil'. I am not sure about the 'black' bit.).
'b'+'oil'='BOIL'

Can you help me to learn more?

(Other definitions for boil that I've seen before include "Painful sore" , "Bubble up - inflamed swelling" , "become inflamed" , "Seethe - painful swelling" , "7 in water" .)

I've seen this clue in The Guardian.

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