Criminal vile and fast in Fiesta (8)
I believe the answer is:
festival
'fiesta' is the definition.
(synonyms)
'criminal vile and fast' is the wordplay.
'criminal' indicates anagramming the letters.
'and' is a charade indicator (letters next to each other).
'vile'+'fast'='vilefast'
'vilefast' with letters rearranged gives 'FESTIVAL'.
'in' acts as a link.
(Other definitions for festival that I've seen before include "Fete" , "Celebratory period" , "Joyful celebration" , "Gala" , "Celebration, religious or artistic" .)