See old boy act as waiter (7)
I believe the answer is:
observe
'see' is the definition.
(I know that observe is the is a more specific form of the action of see.)
'old boy act as waiter' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'act as waiter' becomes 'serve' (I've seen this before).
'ob'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Mark" , "See - say" , "Notice or make a remark" , "Watch, take note of" , "Watch closely" .)