One who doubts cats go in flaps (8)
I believe the answer is:
agnostic
'one who doubts' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'cats go in flaps' is the wordplay.
'flaps' is an anagram indicator (I've seen 'flapping' mean this (I've seen this in other clues)).
'cats'+'go'+'in'='catsgoin'
'catsgoin' with letters rearranged gives 'AGNOSTIC'.
Can you help me to learn more?
(Other definitions for agnostic that I've seen before include "Not sure" , "He finds it difficult to believe" , "Coasting as one who believes the mind cannot know anything of a god" , "Questioning" , "Don't know" .)