The most common and most likely answer to this crossword clue is the 5 letter word SYNTH
I've seen this crossword clue in Financial Times
S
Y
N
T
H
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 = synth.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 SYNTH mean?
SYNTH noun- A musical synthesizer.
- To play on a musical synthesizer.