He's well ahead in anatomy (6)
I believe the answer is:
leader
'he's well ahead in anatomy' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's well ahead in' is the wordplay.
'he' becomes 'Ad'.
'well ahead' becomes 'leer' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'ad' placed inside 'leer' is 'LEADER'.
Can you help me to learn more?
(Other definitions for leader that I've seen before include "Opinion piece; boss" , "Head, guide" , "Head (of government, etc)" , "Director, guide" , "Editorial - chief" .)