In ancient customs rook fell disastrously (8)
I believe the answer is:
folklore
'in ancient customs' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'rook fell disastrously' is the wordplay.
'disastrously' is an anagram indicator (I've seen this in other clues).
'rook'+'fell'='rookfell'
'rookfell' is an anagram of 'FOLKLORE'.
Can you help me to learn more?
(Other definitions for folklore that I've seen before include "Ancient customs and beliefs" , "Stories and legends" , "Stories, traditions, etc., of a group of people" , "Traditional knowledge of a culture" , "Traditional beliefs and tales of community" .)