Fancy dress with it, a failure (8)
I believe the answer is:
disaster
'failure' is the definition.
(I've seen this before)
'fancy dress with it a' is the wordplay.
'fancy' indicates an anagram.
'with' means one lot of letters go next to another.
'dress'+'it'+'a'='dressita'
'dressita' is an anagram of 'DISASTER'.
(Other definitions for disaster that I've seen before include "Event causing great damage" , "Heavy blow" , "terrible trouble" , "Great tradegy or cataclysm" , "Star dies (anag)" .)