Name of old boy, one in service (5)
I believe the answer is:
robin
'name' is the definition.
(name for men or women)
'old boy one in service' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'service' becomes 'RN' (abbreviation for Royal Navy).
'ob'+'i'='obi'
'obi' going within 'rn' is 'ROBIN'.
'of' is the link.
(Other definitions for robin that I've seen before include "See 7" , "So-called outlaw" , "Day was one" , ". . . . . Hood was a medieval outlaw" , "Red and brown songbird" .)