Fantastic fire trick almost failing (8)
I believe the answer is:
terrific
'fantastic' is the definition.
(I know that fantastic can be written as terrific)
'fire trick almost failing' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'failing' indicates an anagram.
'fire'+'trick'='firetrick'
'firetrick' with its last letter removed is 'firetric'.
'firetric' with letters rearranged gives 'TERRIFIC'.
(Other definitions for terrific that I've seen before include "Marvellous, excellent" , "Excellent (colloq.)" , "Wonderful, marvelous" , "Fit crier being excellent" , "'Fantastic, tremendous (8)'" .)