Former exercise left to throw out (5)
I believe the answer is:
expel
'throw out' is the definition.
('expel' can be a synonym of 'throw out')
'former exercise left' is the wordplay.
'former' becomes 'ex'.
'exercise' becomes 'pe' (Physical Education).
'left' becomes 'l' (common abbreviation).
'ex'+'pe'+'l'='EXPEL'
'to' acts as a link.
(Other definitions for expel that I've seen before include "Drive away" , "Eject (from school)" , "Banish (undesirable alien?)" , "Oust" , "Throw out" .)