Low, initially depressed 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)
'low initially depressed' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'initially' indicates taking the first letters.
The initial letter of 'depressed' is 'd'.
'moo'+'d'='MOOD'
(Other definitions for mood that I've seen before include "Humour - bad temper" , "State of mind or emotion" , "feelings" , "Depression" , "Doom (anag)" .)