Favourite pupil taken aback after teacher's blunder (7)
I believe the answer is:
misstep
'blunder' is the definition.
(misstep is a kind of blunder)
'favourite pupil taken aback after teacher's' is the wordplay.
'favourite pupil' becomes 'pet' (I've seen this before).
'taken aback' says the letters should be written backwards.
'after' means one lot of letters go next to another.
'teacher' becomes 'miss' (I've seen this before).
'pet' reversed gives 'tep'.
'tep' put after 'miss' is 'MISSTEP'.
(Other definitions for misstep that I've seen before include "False move" , "bad move" , "Badly judged footing" , "Error" , "Badly judged move" .)