Boil lanced by doctor is serious (6)
I believe the answer is:
sombre
'serious' is the definition.
(I've seen this before)
'boil lanced by doctor' is the wordplay.
'boil' becomes 'sore' (boil can mean a sore or inflammation).
'lanced by' is an insertion indicator (some letters cut or lance into others).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'sore' going around 'mb' is 'SOMBRE'.
'is' acts as a link.
(Other definitions for sombre that I've seen before include "Melancholy" , "Dark, solemn" , "Dim or dismal" , "Dark and gloomy of manner" , "Melancholic" .)