They treat bones and muscles — doctoring to the psoas? (10)
I believe the answer is:
osteopaths
'they treat bones and muscles' is the definition.
(I know this)
'doctoring to the psoas?' is the wordplay.
'doctoring' indicates an anagram.
'to'+'the'+'psoas'='tothepsoas'
'tothepsoas' anagrammed gives 'OSTEOPATHS'.
(Other definitions for osteopaths that I've seen before include "Manipulative types" , "they deal with back issues" , "Bone manipulators" .)