In favour of case that's challenging (11)
I believe the answer is:
provocative
'challenging' is the definition.
(thesaurus)
'in favour of case' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'case' becomes 'vocative' (the vocative case in grammar).
'pro'+'vocative'='PROVOCATIVE'
'that's' is the link.
(Other definitions for provocative that I've seen before include "Designed to cause anger or arousal" , "likely to cause annoyance" , "Deliberately annoying" , "Deliberately causing annoyance" , "Aggravating" .)