The most common and most likely answer to this crossword clue is the 7 letter word CELESTE
I've seen this crossword clue in The Sunday Telegraph
C
E
L
E
S
T
E
Buddy explains!

The straight part
We know that "Instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instrument = celeste.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 CELESTE mean?
CELESTE noun- A musical instrument consisting principally of a set of graduated steel plates struck with hammers that are activated by a keyboard.
- An organ stop, deliberately slightly out of tune to give an undulating sound.