Watch old boy start game in court (7)
I believe the answer is:
observe
'watch' is the definition.
(observing is a kind of watching)
'old boy start game in court' is the wordplay.
'old boy' becomes 'ob'.
'start game in court' becomes 'serve' (serving could begin a game of tennis).
'ob'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Watch without intervening" , "See - keep - remark - maintain - celebrate" , "Mark" , "Look at" , "monitor" .)