Phone old boy missing race once (4)
I believe the answer is:
mile
'race once' is the definition.
(historical term for a mile-long race)
'phone old boy missing' is the wordplay.
'phone' becomes 'mobile phone' (mobile phone).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'missing' suggests deleting specific letters.
'mobile' with 'ob' taken out is 'MILE'.
(Other definitions for mile that I've seen before include "Eight furlongs" , "'A miss is as good as a ....'" , "Stretch of road" , "Distance race" , "Imperial length" .)