Plainchant in service (8)
I believe the answer is:
evensong
This might be a double definition.
'plainchant' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'service' is the second definition.
(type of church service)
'in' acts as a link.
Can you help me to learn more?
(Other definitions for evensong that I've seen before include "6 [SERVICE] ?" , "Service of evening prayer in Church of England" , "Religious service" , "Church service late in the day" , "Vespers" .)