Playing tennis, keeping ball on line is rude (8)
I believe the answer is:
insolent
'rude' is the definition.
(thesaurus)
'playing tennis keeping ball on line' is the wordplay.
'playing' indicates an anagram.
'keeping' means one lot of letters goes inside another.
'ball' becomes 'o'.
'on' means one lot of letters go next to another.
'line' becomes 'l'.
'tennis' anagrammed gives 'insent'.
'o'+'l'='ol'
'insent' placed around 'ol' is 'INSOLENT'.
'is' is the link.
(Other definitions for insolent that I've seen before include "Brash" , "Rude, showing lack of respect" , "Lippy" , "Cheeky" , "Rude and disrespectful" .)