Much such work is done in the head (6)
I believe the answer is:
dental
'much' is the definition.
Although both the answer and definition are adjectives, I can't see how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for dental that I've seen before include "sort of surgeon" , "Pertaining to teeth" , "surgery" , "concerned with choppers" , "Concerned with teeth" .)