Tedious last part filling empty day (6)
I believe the answer is:
dreary
'tedious' is the definition.
(synonyms)
'last part filling empty day' is the wordplay.
'last part' becomes 'rear' (eg the rear of a parade).
'filling' is an insertion indicator.
'empty' suggests removing the centre.
'day' with its middle taken out is 'dy'.
'rear' placed into 'dy' is 'DREARY'.
(Other definitions for dreary that I've seen before include "Gloomy and dismal" , "Dismal, funeral" , "Drab and dismal" , "Gloomy, boring" , "Dismal, depressing" .)