Arranged slam-dance festival (9)
I believe the answer is:
candlemas
'festival' is the definition.
(I have seen 'A festival' mean 'candlemas' so perhaps 'festival' could also mean 'candlemas')
'arranged slam-dance' is the wordplay.
'arranged' is an anagram indicator.
'slam'+'dance'='slamdance'
'slamdance' anagrammed gives 'CANDLEMAS'.
(Other definitions for candlemas that I've seen before include "February Christian festival" , "church festival" , "feast day" , "Feast of the Purification of the Virgin Mary" .)