Look into doctor, say — that's nice to hear (7)
I believe the answer is:
melodic
'that's nice to hear' is the definition.
(something melodic is pleasant to listen to)
'look into doctor' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'into' means one lot of letters goes inside another.
'doctor' becomes 'medic' (doctor is a type of medic).
'lo' put inside 'medic' is 'MELODIC'.
'say' is the link.
(Other definitions for melodic that I've seen before include "Pleasant-sounding" , "Harmonious" , "Pleasant to listen to" , "Old mice (anag.)" , "nice on the ear" .)