Protein supplied by doctor's patient at home (6)
I believe the answer is:
casein
'protein' is the definition.
(casein is a kind of protein)
'doctor's patient at home' is the wordplay.
'doctor's patient' becomes 'case' (case can mean a patient treated by a doctor).
'at home' becomes 'in' (similar in meaning).
'case'+'in'='CASEIN'
'supplied by' is the link.
(Other definitions for casein that I've seen before include "something for cheese-making" , "Protein in milk and cheese" , "It helps to make cheese" , "What's to be found in milk" .)