Grave abscess was a case for the doctor (6)
I believe the answer is:
sombre
'grave abscess' is the definition.
'sombre' can be an answer for 'grave' (I've seen this before). I'm not certain of the 'abscess' bit.
'was a case for the doctor' is the wordplay.
'was' becomes 'sore' (I can't justify this - if you can you should give a lot more credence to this answer).
'a case for' indicates putting letters inside.
'the doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'sore' enclosing 'mb' is 'SOMBRE'.
Can you help me to learn more?
(Other definitions for sombre that I've seen before include "Dark, serious, gloomy" , "Dark, solemn" , "Melancholy" , "Gloomy and dismal" , "Joyless" .)