Thin, about medium, obtaining treatment (6)
I believe the answer is:
remedy
'treatment' is the definition.
(I know that remedy is a type of treatment)
'thin about medium' is the wordplay.
'thin' becomes 'reedy' (as in a thin or reedy voice).
'about' means one lot of letters goes inside another.
'medium' becomes 'm' (as in UK clothing sizes).
'reedy' placed around 'm' is 'REMEDY'.
'obtaining' acts as a link.
(Other definitions for remedy that I've seen before include "Put right, be a cure for" , "Treatment; redress" , "A cure, for disease or other ill" , "Medicine or therapy that cures disease or relieves pain" , "'Cure, medicine (6)'" .)