Stop holding back passion in school (6)
I believe the answer is:
period
'stop' is the definition.
(I know that stop can be written as period)
'back passion in school' is the wordplay.
'back' is a reversal indicator.
'passion' becomes 'ire' (ire is passionate anger).
'in' is an insertion indicator.
'school' becomes 'pod' (term for a group of marine mammals).
'ire' in reverse letter order is 'eri'.
'eri' placed into 'pod' is 'PERIOD'.
'holding' acts as a link.
(Other definitions for period that I've seen before include "Amount of time - US full stop" , "Era, epoch" , "term" , "Stop in America" , "School session" .)