Religious clique or a piece of cake! (6)
I believe the answer is:
sector
'a piece of cake' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'religious clique or' is the wordplay.
'religious clique' becomes 'sect' (sect is a kind of clique. I am not sure about the 'religious' bit.).
'sect'+'or'='SECTOR'
Can you help me to learn more?
(Other definitions for sector that I've seen before include "Military sub-area" , "Distinctive part, of a circle perhaps" , "A body of people who form part of society or economy" , "battle zone" , "Corset (anag) - part of a circle" .)