Right-wing US pal organised debauched feast (9,5)
I believe the answer is:
walpurgis night
'debauched feast' is the definition.
(ancient feast day)
'right-wing us pal organised' is the wordplay.
'organised' indicates anagramming the letters.
'right'+'wing'+'us'+'pal'='rightwinguspal'
'rightwinguspal' anagrammed gives 'WALPURGIS NIGHT'.
(Other definitions for walpurgis night that I've seen before include "30th April?" , "May Day's eve (from German folklore)" , "One evening" , "enchanting event?" , "Witches' celebration" .)