A low-key state of mind (4)
I believe the answer is:
mood
'state of mind' is the definition.
(one's mood is one's state of mind)
'a low-key' is the wordplay.
'a low' becomes 'moo' (both can mean to make the noise of a cow).
'key' becomes 'd' (musical key).
'moo'+'d'='MOOD'
(Other definitions for mood that I've seen before include "Feeling of doom" , "sullenness may be indicative" , "Humour - bad temper" , "at times appears very high" , "State of mind or emotion" .)