Watch old boy put the ball into play (7)
I believe the answer is:
observe
'watch' is the definition.
(I know that watch can be written as observe)
'into play' is the wordplay.
'into' becomes 'ob' (I can't explain this - if you can you should give a lot more credence to this answer).
'play' becomes 'serve' (serve is a kind of play).
'ob'+'serve'='OBSERVE'
'old boy put the ball' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for observe that I've seen before include "Espy" , "Watch closely" , "What this newspaper does?" , "Witness, watch" , "Keep - watch" .)