Tuneful little operetta primarily penned by doctor (7)
I believe the answer is:
melodic
'tuneful' is the definition.
(I've seen this before)
'little operetta primarily penned by doctor' is the wordplay.
'primarily' suggests taking the first letters.
'penned by' indicates putting letters inside (inserted letters are penned in).
'doctor' becomes 'medic' (doctor is a type of medic).
The first letters of 'little operetta' is 'lo'.
'lo' placed into 'medic' is 'MELODIC'.
(Other definitions for melodic that I've seen before include "Old mice (anag.)" , "Pleasantly musical" , "Pleasant-sounding" , "Pleasant-sound" , "Harmonious" .)