Watching old boy performing functions (9)
I believe the answer is:
observing
'watching' is the definition.
('observe' can be a synonym of 'watch')
'old boy performing functions' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'performing functions' becomes 'serving' (I can't explain this - if you can you should believe this answer much more).
'ob'+'serving'='OBSERVING'
Can you help me to learn more?
(Other definitions for observing that I've seen before include "Noticing or perceiving" , "Watching and perhaps taking notes" , "Taking note of" , "Watching attentively" , "Looking at" .)