Cook has painful swelling (4)
I believe the answer is:
boil
This could be a double definition.
'cook' is the first definition.
(boiling is a kind of cooking)
'painful swelling' is the second definition.
'boil' can be an answer for 'swelling' (thesaurus). I am unsure of the 'painful' bit.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for boil that I've seen before include "let off steam" , "Cook in hot water" , "7 in water" , "Sore with a hard core filled with pus" , "Cook, swelling" .)