Musical doctor: see within (7)
I believe the answer is:
melodic
'musical' is the definition.
(I know that musical can be written as melodic)
'doctor see within' is the wordplay.
'doctor' becomes 'medic' (doctor is a type of medic).
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'within' indicates putting letters inside.
'medic' enclosing 'lo' is 'MELODIC'.
(Other definitions for melodic that I've seen before include "Old mice (anag.)" , "Pleasantly musical" , "Harmonious" , "Pleasant-sound" , "nice on the ear" .)