Lose one's temper (3,3,3,6)
I think it could be either:
fly off the handle
'lose one's temper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fly off the handle that I've seen before include "Lose one's temper" , "Robin perching on spade may" , "See red" , "react angrily" .)
'lose one's temper' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for see red that I've seen before include "Lose temper as bull might" , "Lose temper colourfully, as bull does (3.3)" , "Lose temper like enraged bull" , "Grow furious" , "Burst a blood vessel" .)