Outrageous fat Elvis celebration (8)
I believe the answer is:
festival
'celebration' is the definition.
(I know that celebration can be written as festival)
'outrageous fat elvis' is the wordplay.
'outrageous' is an anagram indicator (I've seen this in other clues).
'fat'+'elvis'='fatelvis'
'fatelvis' with letters rearranged gives 'FESTIVAL'.
(Other definitions for festival that I've seen before include "Day set aside for celebration" , "Gala" , "Period for feasting and celebration" , "Lift vase (anag.)" , "Holy day" .)