The most common and most likely answer to this crossword clue is the 11 letter word VIOLONCELLO
I've seen this crossword clue in The Times
V
I
O
L
O
N
C
E
L
L
O
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 = violoncello.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 VIOLONCELLO mean?
VIOLONCELLO noun- A large stringed instrument of the violin family, but smaller than the double bass.