Fail to observe religious celebration (8)
I believe the answer is:
passover
'fail' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for passover that I've seen before include "feast - with 25a" , "spring festival" , "religious festival" , "Disregard - Jewish festival" .)