Material use for musical strings, made from intestines of sheep or horses (but not cats) (6)
I believe the answer is:
catgut
(Other definitions for catgut that I've seen before include "Material used for violin strings, for example" , "'Strong cord, used in surgery and musical instruments (6)'" , "Tough string for stitches and musical instruments" , "Strong cord for stitches, etc" , "Material in violin string" .)