Discharge erstwhile gym class learner (5)
I believe the answer is:
expel
'discharge' is the definition.
(I know that discharge can be written as expel)
'erstwhile gym class learner' is the wordplay.
'erstwhile' becomes 'ex' (prefix meaning former or onetime).
'gym class' becomes 'pe' (abbreviation for physical education).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'ex'+'pe'+'l'='EXPEL'
(Other definitions for expel that I've seen before include "Get rid" , "Drive out, banish" , "bar" , "Reject" , "Boot out" .)