Start one's performance on time (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'one's performance on time' is the wordplay.
'performance' is an anagram indicator.
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'ones' with letters rearranged gives 'onse'.
'onse'+'t'='ONSET'
(Other definitions for onset that I've seen before include "Inception" , "Stone (anag)" , "Start - attack" , "starting?" , "Beginning; military attack" .)