Serious wound is plugged by doctor (6)
I believe the answer is:
sombre
'serious' is the definition.
(I've seen this before)
'wound is plugged by doctor' is the wordplay.
'wound' becomes 'sore' (I've seen this before).
'is plugged by' means one lot of letters goes inside another (plug can mean to block up a gap).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'sore' enclosing 'mb' is 'SOMBRE'.
(Other definitions for sombre that I've seen before include "Gloomy and dismal" , "Grave or gloomy" , "Dark, solemn" , "Melancholic" , "Dim or dismal" .)