The most common and most likely answer to this crossword clue is the 6 letter word CATGUT
I've seen this crossword clue in Financial Times
C
A
T
G
U
T
More clues leading to the result CATGUT
Buddy explains!

The straight part
We know that "What bassists use" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what bassists use = catgut.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 CATGUT mean?
CATGUT noun- A cord of great toughness made from the intestines of animals, especially of sheep, used for strings of musical instruments, etc.
- The material from which such cords are made.
- A sort of linen or canvas, with wide interstices.