Fat cats in trouble? Great! (9)
I believe the answer is:
fantastic
'great' is the definition.
(thesaurus)
'fat cats in trouble?' is the wordplay.
'trouble?' indicates anagramming the letters (I've seen 'of trouble' mean this (I've seen this in other clues)).
'fat'+'cats'+'in'='fatcatsin'
'fatcatsin' with letters rearranged gives 'FANTASTIC'.
(Other definitions for fantastic that I've seen before include "Terrific" , "Great!" , "Super!" , "Brilliant or unbelievable" , "Wild" .)