Watch old boy start a game of tennis? (7)
I believe the answer is:
observe
'watch' is the definition.
(observing is a kind of watching)
'old boy start a game of tennis?' is the wordplay.
'old boy' becomes 'ob'.
'start a game of tennis?' becomes 'serve'.
'ob'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Abide by -- watch" , "Keep - watch" , "Notice, watch" , "Notice, perceive" , "Notice; study" .)