The most common and most likely answer to this crossword clue is the 5 letter word CHANT
I've seen this crossword clue in The Daily Telegraph
C
H
A
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Buddy explains!

The straight part
We know that "plainsong" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plainsong = chant.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 CHANT mean?
CHANT noun- Type of singing done generally without instruments and harmony.
- A short and simple melody, divided into two parts by double bars, to which unmetrical psalms, etc., are sung or recited. It is the most ancient form of choral music.
- Twang; manner of speaking; a canting tone.
- A repetitive song, typically an incantation or part of a ritual.
- To sing, especially without instruments, and as applied to monophonic and pre-modern music.
- To sing or intone sacred text.
- To utter or repeat in a strongly rhythmical manner, especially as a group.
- To sell horses fraudulently, exaggerating their merits.