Infection has doctor in dark (6)
I believe the answer is:
sombre
'dark' is the definition.
(I've seen this before)
'infection has doctor' is the wordplay.
'infection' becomes 'sore' (sore is a kind of infection).
'has' indicates putting letters inside.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'sore' placed around 'mb' is 'SOMBRE'.
'in' is the link.
(Other definitions for sombre that I've seen before include "Dark, serious, gloomy" , "Melancholic" , "Dim or dismal" , "Funereal" , "Gloomy and dismal" .)