Suture holds in tendon (5)
I believe the answer is:
sinew
'tendon' is the definition.
(synonyms)
'suture holds in' is the wordplay.
'suture' becomes 'sew' (I've seen this in another clue).
'holds' indicates putting letters inside.
'sew' going around 'in' is 'SINEW'.
(Other definitions for sinew that I've seen before include "source of physical strength" , "Gristle" , "Muscular strength" , "Cord of tissue connecting muscle to bone" , "Tendon - ligament" .)