Agree to catch up, showing sinew (6)
I believe the answer is:
tendon
'sinew' is the definition.
('tendon' can be a synonym of 'sinew')
'agree to catch up' is the wordplay.
'agree' becomes 'nod' (I've seen this before).
'to catch' becomes 'net' (netting is a kind of catching).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'nod'+'net'='nodnet'
'nodnet' backwards is 'TENDON'.
'showing' is the link.
(Other definitions for tendon that I've seen before include "Muscle attacher" , "personal connecting link" , "Sinew, maybe 3" , "Tissue cord connecting muscle to bone" , "In words" .)