A low-key state of mind (4)
I believe the answer is:
mood
'state of mind' is the definition.
(I've seen this before)
'a low-key' is the wordplay.
'a low' becomes 'moo' (I've seen this before).
'key' becomes 'd' (I've seen this in other clues**).
'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" .)