Eastenders in churches concealing large mistakes (5)

I believe the answer is:
apses
'mistakes' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for apses that I've seen before include "Parts of church" , "Does 'e pass these in churches?" , "Domed areas in church, usually where the altar is" , "Domed areas in church, usually where the altars are" , "Domed areas in churches" .)
