Childish outburst (7)
I believe the answer is:
tantrum
'childish outburst' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tantrum that I've seen before include "Child's outburst of bad temper" , "Childish outbreak of temper" , "Burst of temper" , "Childish display of temper" , "Uncontrolled outburst of anger" .)