Sinew died in part of joint (6)
I believe the answer is:
'sinew' is the definition.
('tendon' can be a synonym of 'sinew')
'died in part of joint' is the wordplay.
'died' becomes 'd'.
'in' is an insertion indicator.
'part of joint' becomes 'tenon' (I've seen this in other clues).
'd' placed inside 'tenon' is 'TENDON'.
(Other definitions for tendon that I've seen before include "Muscle-bone link" , "It's attaches tissue to bone" , "Connective tissue" , "Tissue cord connecting muscle to bone" , "Tissue connecting a muscle with its bony attachment" .)