If backed at Ascot almost a complete failure (6)
I believe the answer is:
fiasco
'a complete failure' is the definition.
(I've seen this before)
'if backed at ascot almost' is the wordplay.
'backed' says the letters should be written backwards.
'at' is a charade indicator (letters next to each other).
'almost' means to remove the last letter (most of the word but not all of it).
'if' backwards is 'fi'.
'ascot' with its last letter removed is 'asco'.
'fi'+'asco'='FIASCO'
(Other definitions for fiasco that I've seen before include "Complete and disastrous failure" , "Calamity" , "Sudden and complete collapse" , "A ludicrous or humiliating failure, complete mess" , "Farcical disaster" .)