Unpleasant behaviour as tennis star's beginning to flip (9)
I believe the answer is:
nastiness
'unpleasant behaviour' is the definition.
(I've seen this before)
'as tennis star's beginning to flip' is the wordplay.
'beginning to' indicates taking the first letters.
'flip' is an anagram indicator (I've seen 'flips' mean this).
The initial letter of 'stars' is 's'.
'as'+'tennis'+'s'='astenniss'
'astenniss' is an anagram of 'NASTINESS'.
(Other definitions for nastiness that I've seen before include "ill humour" , "Repugnant behaviour" , "Pollution" , "hostility" , "Unpleasant behaviour" .)