A 'low key' frame of mind (4)
I believe the answer is:
mood
'frame of mind' is the definition.
(I've seen this in another clue)
'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 "State of mind or feeling" , "at times appears very high" , "sullenness may be indicative" , "State of mind, humour" , "feelings" .)