Sinew is key in part of joint (6)
I believe the answer is:
tendon
'sinew' is the definition.
(synonyms)
'key in part of joint' is the wordplay.
'key' becomes 'd' (musical key).
'in' means one lot of letters goes inside another.
'part of joint' becomes 'tenon' (type of wooden joint).
'd' inserted inside 'tenon' is 'TENDON'.
'is' acts as a link.
(Other definitions for tendon that I've seen before include "Ligament - sinew" , "Strong band of tissue connecting muscles" , "Fibrous band" , "Tissue connecting a muscle with its bony attachment" , "Fibrous tissue attaching muscle to bone" .)