Doctor keeping watch? That’s good to hear! (7)
I believe the answer is:
'good to hear' is the definition.
(I've seen this in another clue)
'doctor keeping watch?' is the wordplay.
'doctor' becomes 'medic' (doctor is a type of medic).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'watch?' becomes 'lo' (archaic exclamation meaning 'look!').
'medic' enclosing 'lo' is 'MELODIC'.
'that's' acts as a link.
(Other definitions for melodic that I've seen before include "Pleasant-sound" , "Of music, tuneful and sweet" , "Old mice (anag.)" , "Harmonious" , "Pleasant to listen to" .)