meltdown (7)
I believe the answer is:
tantrum
'meltdown' is the definition.
(I know that meltdown can be written as tantrum)
This is all the clue.
(Other definitions for tantrum that I've seen before include "Childish outbreak of temper" , "Frenzy" , "Burst of temper" , "Hissy fit" , "Childish fit of rage" .)