Mournful medic lancing boil? (6)
I believe the answer is:
sombre
'mournful' is the definition.
(thesaurus)
'medic lancing boil?' is the wordplay.
'medic' becomes 'MB' (Bachelor of Medicine).
'lancing' is an insertion indicator (some letters cut or lance into others).
'boil?' becomes 'sore' (boil can mean a sore or inflammation).
'mb' going into 'sore' is 'SOMBRE'.
(Other definitions for sombre that I've seen before include "Dark and dismal" , "Melancholy" , "Dark, solemn" , "Funereal" , "Dark and gloomy of manner" .)