Schoolgirl tantrum about lapse of judgment (7)
I believe the answer is:
Here is my best explanation:
'lapse of judgment' is the definition.
'schoolgirl tantrum about' is the subsidiary indication.
'schoolgirl' becomes 'miss' (I've seen this before).
'tantrum' becomes 'pet'.
'about' says the letters should be written backwards.
'pet' written backwards gives 'tep'.
(Other definitions for misstep that I've seen before include "False move", "Error", "indiscretion", "bad move", "Badly judged move" and "Badly judged footing".)