Jewish Day of Atonement (3,6)
I believe the answer is:
yom kippur
'jewish day of atonement' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for yom kippur that I've seen before include "Jewish solelmn Day of Atonement" , "Annual solemn Jewish Day of Atonement" , "Jewish Day of Atonement, most solemn of the year" , "Jewish holiday, also called Day of Atonement" , "Religious fast" .)