Schoolgirl tantrum about lapse of judgment (7)
I believe the answer is:
misstep
'lapse of judgment' is the definition.
'schoolgirl tantrum about' is the wordplay.
'schoolgirl' becomes 'miss' (I've seen this before).
'tantrum' becomes 'pet'.
'about' says the letters should be written in reverse.
'pet' written backwards gives 'tep'.
'miss'+'tep'='MISSTEP'
(Other definitions for misstep that I've seen before include "Error" , "bad move" , "False move" , "Badly judged move" , "Badly judged footing" .)