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' means one lot of letters goes inside another.
'sew' placed 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" .)