Fibrous tissue beginning to decay in joint (6)
I believe the answer is:
tendon
'fibrous tissue' is the definition.
(tendon is a kind of fibrous tissue)
'beginning to decay in joint' is the wordplay.
'beginning to' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'joint' becomes 'tenon' (I've seen this before).
The initial letter of 'decay' is 'd'.
'd' put inside 'tenon' is 'TENDON'.
(Other definitions for tendon that I've seen before include "Sinew, maybe 3" , "Strong cord of bodily tissue" , "Connective tissue" , "personal connecting link" , "Strong band of tissue connecting muscles" .)