Watch old boy start tennis game (7)
I believe the answer is:
observe
'watch' is the definition.
(I know that watch can be written as observe)
'old boy start tennis game' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (abbreviation) and 'o' is found in the answer.
'boy' could be 'erv' and 'erv' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for observe that I've seen before include "Watch closely" , "Look at" , "taking notice" , "Notice or make a remark" , "Abide by -- watch" .)