Low key state of mind (4)
I believe the answer is:
mood
'state of mind' is the definition.
(I've seen this before)
'low key' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'key' becomes 'd' (I've seen this in other clues**).
'moo'+'d'='MOOD'
(Other definitions for mood that I've seen before include "Frame of mind" , "at times appears very high" , "Feeling of doom" , "State of mind, humour" , "feelings" .)