The most common and most likely answer to this crossword clue is the 8 letter word GRAPHEME
G
R
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P
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M
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More clues leading to the result GRAPHEME
Buddy explains!

The straight part
We know that "Symbol to represent sound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case symbol to represent sound = grapheme.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 GRAPHEME mean?
GRAPHEME noun- A fundamental unit of a writing system, corresponding to (for example) letters in the English alphabet or jamo in Korean Hangeul.
- In alphabetic writing, the shortest group of letters composing a phoneme.