Sombre listener in flat (6)
I believe the answer is:
dreary
'sombre' is the definition.
(thesaurus)
'listener in flat' is the wordplay.
'listener' becomes 'ear' (body part which hears or listens).
'in' is an insertion indicator.
'flat' becomes 'dry' (both can mean dull).
'ear' inserted within 'dry' is 'DREARY'.
(Other definitions for dreary that I've seen before include "routine" , "Gloomy - dull" , "Banal" , "Dull and depressing" , "Gloomy and dismal" .)