If backed at Ascot almost a complete failure (6)
I believe the answer is:
Here is my best explanation:
'a complete failure' is the definition.
(I've seen this before)
'if backed at ascot almost' is the subsidiary indication.
'backed' says the letters should be written in reverse.
'at' says to put letters next to each other.
'almost' means to remove the last letter (most of the word but not all of it).
'if' in reverse letter order is 'fi'.
'ascot' with its last letter taken away is 'asco'.
(Other definitions for fiasco that I've seen before include "Disaster", "A debacle, complete mess", "An utter mess or cock-up", "A ludicrous or humiliating failure, complete mess", "A ludicrous failure, a shambles", "A disaster", "Embarrassing failure", "An absolute mess, what a farce", "Ignominious result" and "A hopeless mess or ludicrous failure".)