One's treated - one who was patient in cosmetic surgery (4,3)
I believe the answer is:
nose job
'cosmetic surgery' is the definition.
(I know that nose job is a type of cosmetic surgery)
'one's treated one who was patient' is the wordplay.
'treated' indicates anagramming the letters.
'one who was patient' becomes 'job' (famously patient Old Testament figure).
'ones' anagrammed gives 'nose'.
'nose'+'job'='NOSE JOB'
'in' acts as a link.
(Other definitions for nose job that I've seen before include "Surgery of the face" , "Assignment for features editor" , "Rhinoplasty" , "Schnozzle surgery" , "Piece of plastic surgery" .)