Possessive person who'll succeed on time (5)
I believe the answer is:
their
'possessive' is the definition.
(I've seen this before)
'person who'll succeed on time' is the wordplay.
'person who'll succeed' becomes 'heir'.
'on' is a charade indicator (letters next to each other).
'time' becomes 't'.
'heir' put after 't' is 'THEIR'.
(Other definitions for their that I've seen before include "Some would say his (or her)" , "belonging to an individual" , "Belonging to him (perhaps wrongly)" , "Those people's" , "Possessive pronoun" .)