A northern beast upended joint (5)
I believe the answer is:
ankle
'joint' is the definition.
(ankle joint is a kind of joint)
'a northern beast upended' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'beast' becomes 'elk' (I've seen this before).
'upended' is a reversal indicator (in a down clue, letters go up).
'elk' back-to-front is 'kle'.
'a'+'n'+'kle'='ANKLE'
(Other definitions for ankle that I've seen before include "Part of the leg between the foot and the shin" , "Bone over foot" , "Major foot joint" , "Foot-leg connector" , "Narrow part of leg" .)