Obedience school no-no (3)
I believe the answer is:
nip
'obedience school no-no' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nip that I've seen before include "Pinch; quantity of spirits" , "Dram of spirits" , "Swift bite" , "(Give) a small sharp bite" , "Pinch; bite" .)