The most common and most likely answer to this crossword clue is the 5 letter word GAMBA
I've seen this crossword clue in Independent
G
A
M
B
A
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 = gamba.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 GAMBA mean?
GAMBA noun- A rank of organ pipes, so called for a supposed resemblance of the sound to that of a viola da gamba.
- A stringed instrument related to the violin family, but held in the lap between the legs like a cello, usually with C-holes, a flat back, a fretted neck and six strings, played with an underhanded bow hold.
- The metacarpus or metatarsus of ruminants, etc.