Tennis racket string (6)

I believe the answer is:

catgut

(Other definitions for catgut that I've seen before include "Natural suture cord now largely replaced by artificial polymer" , "Natural material for violin strings" , "Strong cord for stitches, etc" , "Tough string for stitches and musical instruments" , "'Strong cord, used in surgery and musical instruments (6)'" .)

I've seen this clue in The Mirror.

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