To be old is to have time (5)
I believe the answer is:
exist
'to be' is the definition.
('exist' can be a synonym of 'be')
'old is to have time' is the wordplay.
'old' becomes 'ex'.
'to have' is a charade indicator (letters next to each other).
'time' becomes 't'.
'ex'+'is'+'t'='EXIST'
(Other definitions for exist that I've seen before include "Get a life!" , "Survive" , "Subsist" , "Have being or continue living" , "Be alive" .)