This physician returned cash on delivery (3)
I believe the answer is:
doc
'this physician' is the definition.
('doc' can be a synonym of 'physician')
'returned cash on delivery' is the wordplay.
'returned' is a reversal indicator.
'cash on delivery' becomes 'cod' ('COD' can be a synonym of 'cash on delivery').
'cod' written backwards gives 'DOC'.
(Other definitions for doc that I've seen before include "Main dish" , "One of the seven dwarfs" , "GP (colloq.)" , "Consultant" , "Medical practitioner (abbr)" .)