Fit rival? Not disheartened (9)
I believe the answer is:
competent
'fit' is the definition.
(thesaurus)
'rival? not disheartened' is the wordplay.
'rival?' becomes 'compete' (rivalling is a kind of competing**).
'disheartened' means to remove the middle letters.
'not' with its middle taken out is 'nt'.
'compete'+'nt'='COMPETENT'
(Other definitions for competent that I've seen before include "Able (to do something)" , "Qualified" , "Teen drink" , ""not bad"" , "Efficient, capable" .)