Low, initially depressed state of mind (4)
I believe the answer is:
Here is my best explanation:
'state of mind' is the definition.
(I've seen this before)
'low initially depressed' is the subsidiary indication.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'initially' suggests taking the first letters.
The initial letter of 'depressed' is 'd'.
(Other definitions for mood that I've seen before include "Humour - bad temper", "melancholy", "feelings", "State of mind, humour", "frame", "An atmosphere of doom", "Disposition", "Depression", "A feeling of doom" and "sullenness may be indicative".)