Old boy to wait on and watch (7)
I believe the answer is:
observe
'watch' is the definition.
(I know that watch can be written as observe)
'old boy to wait on' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'to wait on' becomes 'serve' (to wait on someone is to serve them).
'ob'+'serve'='OBSERVE'
'and' is the link.
(Other definitions for observe that I've seen before include "Keep an eye on" , "Watch attentively" , "Keep - watch" , "monitor" , "Scrutinise or comply with" .)