Sick cat faints? That’s bizarre (9)
I believe the answer is:
fantastic
'bizarre' is the definition.
(thesaurus)
'sick cat faints?' is the wordplay.
'sick' indicates an anagram.
'cat'+'faints'='catfaints'
'catfaints' anagrammed gives 'FANTASTIC'.
'that's' is the link.
(Other definitions for fantastic that I've seen before include "Extraordinarily good, tremendous" , "Brilliant or unbelievable" , "Marvellous" , "Wild" , "Illusory - excellent!" .)