Get hot under the collar (6)
I believe the answer is:
seethe
'get hot under the collar' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seethe that I've seen before include "Bubble as in boiling, or be very angry" , "Be silently angry" , "Boil; be crowded with moving people" , "Boil or become very angry" , "Feel intense rage" .)