It's implausible for fat cats in compound (9)
I believe the answer is:
fantastic
'it's implausible' is the definition.
(thesaurus)
'fat cats in compound' is the wordplay.
'compound' indicates an anagram.
'fat'+'cats'+'in'='fatcatsin'
'fatcatsin' with letters rearranged gives 'FANTASTIC'.
'for' acts as a link.
(Other definitions for fantastic that I've seen before include "Great!" , "Brilliant or unbelievable" , "Fanciful" , "Extraordinarily good, tremendous" , "Weird" .)