The most common and most likely answer to this crossword clue is the 10 letter word PLAINCHANT
I've seen this crossword clue in Financial Times
P
L
A
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Buddy explains!
The straight part
We know that "sung in church" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sung in church = plainchant.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PLAINCHANT mean?
PLAINCHANT noun- A form of monophonic chant in unison using the Gregorian scale, sung in various Christian churches.
- A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song.
- The simple notes of an air, without ornament or variation.
- (by extension) A plain unvarnished statement, without exaggeration.