Perceptive old boy, one below stairs, perhaps (9)
I believe the answer is:
observant
'perceptive' is the definition.
(similar in meaning)
'old boy one below stairs perhaps' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'one below stairs perhaps' becomes 'servant' (servants traditionally had downstairs quarters).
'ob'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "Sharp-eyed" , "keeping close eye on things" , "Quick to notice things" , "Watchful" , "Alert" .)