Teacher rebels against bullying by insensitive principals (5)
I believe the answer is:
rabbi
'teacher' is the definition.
(I've seen this before)
'rebels against bullying by insensitive principals' is the wordplay.
'principals' indicates taking the first letters.
The first letters of 'rebels against bullying by insensitive' is 'RABBI'.
(Other definitions for rabbi that I've seen before include "Religious person" , "A Jewish clergyman" , "Hebrew scholar" , "Sacks, for example" , "A Jewish priest" .)