Number observed to hold race, perhaps (9)
I believe the answer is:
seventeen
'number' is the definition.
(seventeen is a kind of number)
'observed to hold race perhaps' is the wordplay.
'observed' becomes 'seen' ('seen' can be a synonym of 'observed').
'to hold' is an insertion indicator.
'race perhaps' becomes 'event' (race is a kind of event).
'seen' enclosing 'event' is 'SEVENTEEN'.
(Other definitions for seventeen that I've seen before include "XVII" , "odd number" , "Cardinal number" , "Quite a few" , "Age when full driving licence can be obtained" .)