This joint is below ground (5)
I believe the answer is:
elbow
'this joint' is the definition.
(joint of the body)
'below ground' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'below' anagrammed gives 'ELBOW'.
'is' is the link.
(Other definitions for elbow that I've seen before include "Arm joint, more power to it" , "Dismissal?" , "sharp turn" , "Barge" , "Jostle" .)