Dull day’s end is sign excluding night’s beginning (10)
I believe the answer is:
drearisome
'dull' is the definition.
(thesaurus)
'day's end is sign excluding night's beginning' is the wordplay.
I cannot really see how this works, but
'end' could be 'rear' (both can mean the back part of something) and 'rear' is found in the answer.
'is' is present in the answer.
'day' could be 'd' (abbreviation for day) and 'd' is present in the answer.
The remaining letters 'ome' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for drearisome that I've seen before include "Lonely, desolate" , "Sad" .)