Promise to include perhaps Milton Keynes's last regressive therapist (9)
I believe the answer is:
osteopath
'therapist' is the definition.
(osteopath is a kind of therapist)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for osteopath that I've seen before include "Health professional" , "Health worker" , "Bone manipulator" , "Specialist manipulator of bones and muscles" , "Potato she uses badly as a manipulator of the body" .)