Boil, foam (6)
I believe the answer is:
seethe
This clue is a double definition.
'boil' is the first definition.
(I know that seethe is a type of boil)
'foam' is the second definition.
(I know that seethe is a type of foam)
(Other definitions for seethe that I've seen before include "Ferment, sizzle" , "Smoulder" , "Boil turbulently" , "Barely contain one's anger" , "Boil, be turbulent" .)