William defending Kitty and Dick endlessly is a joke (9)
I believe the answer is:
witticism
'a joke' is the definition.
(thesaurus)
'william defending kitty and dick endlessly is' is the wordplay.
I cannot quite understand how this works, but
'is' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for witticism that I've seen before include "Sparkling remark" , "Quip" , "Clever remark" .)