Achilles being one poorly noted name (6)
I believe the answer is:
tendon
'achilles' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one poorly noted name' is the wordplay.
'one poorly' indicates an anagram (a poor spelling).
'name' becomes 'n' (common abbreviation e.g. on forms).
'noted' is an anagram of 'tendo'.
'tendo'+'n'='TENDON'
'being' is the link.
Can you help me to learn more?
(Other definitions for tendon that I've seen before include "Tissue cord connecting muscle to bone" , "Ligament - sinew" , "horse's hamstring" , "Strong band of tissue connecting muscles" , "It's attaches tissue to bone" .)