Agree to catch up, showing sinew (6)
I believe the answer is:
tendon
'sinew' is the definition.
(I know that sinew can be written as tendon)
'agree to catch up' is the wordplay.
'agree' becomes 'nod' (I've seen this before).
'to catch' becomes 'net' (net can mean to catch).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'nod'+'net'='nodnet'
'nodnet' backwards is 'TENDON'.
'showing' acts as a 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" .)