Fiats can't crash? That's unbelievable! (9)
I believe the answer is:
fantastic
'unbelievable' is the definition.
(thesaurus)
'fiats can't crash?' is the wordplay.
'crash?' is an anagram indicator (I've seen 'crashed' mean this).
'fiats'+'cant'='fiatscant'
'fiatscant' anagrammed gives 'FANTASTIC'.
'that's' acts as a link.
(Other definitions for fantastic that I've seen before include "Splendid, excellent" , "Wild" , "Brilliant or unbelievable" , "Illusory - excellent!" , "Great!" .)