Simmer with anger (6)
I believe the answer is:
seethe
'simmer with anger' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seethe that I've seen before include "Boil turbulently" , "Barely contain one's anger" , "Appear as if boiling" , "become agitated" , "Ferment, sizzle" .)