Material for violin strings (6)
I believe the answer is:
'material for violin strings' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for catgut that I've seen before include "Traditional violin string" , "Strong cord used in surgery and violin strings" , "Material used for violin strings, for example" , "Strong cord for stitches, etc" , "Natural material for violin strings" .)