Sullen mood, ours? Not entirely (4)
I believe the answer is:
dour
'sullen' is the definition.
(I know that sullen can be written as dour)
'mood ours? not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'DOUR' is hidden in the letters of 'mood ours'.
(Other definitions for dour that I've seen before include "Morose, sour" , "Uncompromising" , "Severe, stern, gloomy" , "Gloomy in manner or appearance" , "Of appearance, harsh and forbidding" .)