Keen-eyed old boy and man (9)
I believe the answer is:
observant
'keen-eyed' is the definition.
(I've seen this before)
'old boy and man' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'and' says to put letters next to each other.
'man' becomes 'servant' (man can mean someone's personal servant).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "Keen-eyed" , "Good at noticing" , "Alert" , "Closely watching" , "attentive to detail" .)