Old exercise left to remove (5)
I believe the answer is:
expel
'remove' is the definition.
(I know that expel is a more specific form of the action remove)
'old exercise left' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'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 "Get rid" , "Banish (undesirable alien?)" , "Oust" , "Spew" , "Drive out - dispossess" .)