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

The straight part
We know that "string instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case string instrument = zither.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 ZITHER mean?
ZITHER noun- A musical instrument consisting of a flat sounding box with numerous strings placed on a horizontal surface, played with a plectrum or fingertips.
- (translations) Related or similar instruments in other cultures, such as the Chinese guqin or Norwegian harpeleik; especially any chordophone without a neck, and with strings that pass over the body.
- To play a zither.
- To make a sound similar to that made by a zither; to move while making such a sound.