Watch the old boy hit the ball first (7)
I believe the answer is:
observe
'watch' is the definition.
(I know that watch can be written as observe)
'old boy hit the ball first' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'hit the ball first' becomes 'serve' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ob'+'serve'='OBSERVE'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for observe that I've seen before include "Mark" , "Espy" , "What this newspaper does?" , "Spot, watch" , "Witness, watch" .)