The most common and most likely answer to this crossword clue is the 4 letter word UKES
I've seen this crossword clue in The Times
U
K
E
S
Buddy explains!

The straight part
We know that "instruments" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruments = ukes.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 UKES mean?
UKES noun- The training partner against whom tori performs a move.
- (Japanese fiction) A passive or submissive male fictional character in a same-sex relationship; a bottom.
- A small four-stringed guitar.