Grand Prix race track in Berkshire unfinished? What a debacle! (6)
I believe the answer is:
fiasco
'what a debacle' is the definition.
(synonyms)
'grand prix race track in berkshire unfinished?' is the wordplay.
'grand prix' becomes 'fi' (Grands Prix are held in F1 or Formula 1 racing).
'race track in berkshire' becomes 'ascot' (horse racecourse).
'unfinished?' means to remove the last letter (I've seen 'unfinished' mean this).
'ascot' with its final letter removed is 'asco'.
'fi'+'asco'='FIASCO'
(Other definitions for fiasco that I've seen before include "An utter mess or cock-up" , "Utter and ridiculous failure" , "washout" , "bomb" , "Total cock-up" .)