Incomplete Oriental holiday (6)
I believe the answer is:
'holiday' is the definition.
(I've seen this before)
'incomplete oriental' is the wordplay.
'incomplete' means to remove the last letter.
'oriental' becomes 'eastern' ('eastern' can be similar in meaning to 'oriental').
'eastern' with its final letter taken away is 'EASTER'.
(Other definitions for easter that I've seen before include "Christian festival in Spring" , "Time to celebrate" , "Christian celebration of the Resurrection" , "Christian celebration at the end of Lent" , "Festival after Lent" .)