Reporter of outside broadcast on tennis player (8)
I believe the answer is:
observer
'reporter' is the definition.
'outside broadcast on tennis player' is the wordplay.
'outside broadcast' becomes 'ob'.
'on' is a charade indicator (letters next to each other).
'tennis player' becomes 'server' (someone who serves).
'ob'+'server'='OBSERVER'
'of' acts as a link.
(Other definitions for observer that I've seen before include "Watcher, spectator" , "organ on Sunday" , "Someone watching" , "Sunday newspaper" , "One who watches" .)