Mood displayed by a doctor having treated niece (8)
I believe the answer is:
ambience
'mood' is the definition.
(ambience is a kind of mood)
'a doctor having treated niece' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'having' means one lot of letters go next to another.
'treated' indicates anagramming the letters.
'niece' with letters rearranged gives 'ience'.
'a'+'mb'+'ience'='AMBIENCE'
'displayed by' acts as a link.
(Other definitions for ambience that I've seen before include "Character and atmosphere of a place" , "Mood of place" , "Surroundings or atmosphere" , "Suitable surroundings for manic bee" , "Atmosphere, aura" .)