Joint an ungulate turned (5)
I believe the answer is:
ankle
'joint' is the definition.
(ankle is a kind of joint)
'an ungulate turned' is the wordplay.
'ungulate' becomes 'elk' (elk is a kind of ungulate).
'turned' shows that the letters should be reversed in order.
'elk' in reverse letter order is 'kle'.
'an'+'kle'='ANKLE'
(Other definitions for ankle that I've seen before include "Human joint" , "Low joint" , "Bone over foot" , "Major foot joint" , "Narrow part of leg" .)