An old one is presumably right on occasion (5)
I believe the answer is:
timer
'an old one is presumably' is the definition.
I can't judge whether this defines the answer.
'right on occasion' is the wordplay.
'right' becomes 'r' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'occasion' becomes 'time' (I've seen this before).
'r' put after 'time' is 'TIMER'.
Can you help me to learn more?
(Other definitions for timer that I've seen before include "Watch, for example" , "Stopwatch, say" , "One may be alarmed" , "Preset activator" , "Preset mechanisation to activate a device" .)