By its very nature top fiasco is disastrous (4,5)
I believe the answer is:
ipso facto
'by its very nature' is the definition.
(Latin for 'by that very fact' ie intrinsically)
'top fiasco is disastrous' is the wordplay.
'is disastrous' is an anagram indicator.
'top'+'fiasco'='topfiasco'
'topfiasco' anagrammed gives 'IPSO-FACTO'.
(Other definitions for ipso facto that I've seen before include "by the very nature of the case)" , "Self-evidently; thereby" , "By that very act (Lat.)" , "For that precise reason" , "By that truth (Latin)" .)