Watching old boy waiting (9)
I believe the answer is:
observing
'watching' is the definition.
(observing is a kind of watching)
'old boy waiting' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'waiting' becomes 'serving' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ob'+'serving'='OBSERVING'
Can you help me to learn more?
(Other definitions for observing that I've seen before include "Taking note of" , "Watching and perhaps taking notes" , "Looking at" , "Watching attentively" , "Noticing or perceiving" .)