The most common and most likely answer to this crossword clue is the 6 letter word QUAVER
Q
U
A
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More clues leading to the result QUAVER
Buddy explains!

The straight part
We know that "shrill sound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shrill sound = quaver.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 QUAVER mean?
QUAVER noun- A trembling shake.
- A trembling of the voice, as in speaking or singing.
- An eighth note, drawn as a crotchet (quarter note) with a tail.
- To shake in a trembling manner.
- To use the voice in a trembling manner, as in speaking or singing.
- To utter quaveringly.