Teacher retires after berating brutally incompetent heads (5)
I believe the answer is:
rabbi
'teacher' is the definition.
(Jewish teacher)
'retires after berating brutally incompetent heads' is the wordplay.
'heads' suggests taking the first letters.
The initial letters of 'retires after berating brutally incompetent' is 'RABBI'.
(Other definitions for rabbi that I've seen before include "A Jewish priest" , "Jewish religious teacher" , "Religious leader in a Jewish community" , "Religious leader of Jewish congregation" , "Spiritual leader of a Jewish congregation" .)