Bill's partner visited by doctor in group (5)
I believe the answer is:
combo
'group' is the definition.
(a combo can mean a musical group)
'bill's partner visited by doctor' is the wordplay.
'bill's partner' becomes 'coo' (I've seen this in another clue).
'visited by' is an insertion indicator.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'coo' enclosing 'mb' is 'COMBO'.
'in' is the link.
(Other definitions for combo that I've seen before include "musicians" , "Small jazz or dance band" , "Small (jazz or rock) band" , "Music group" , "group of players" .)