Feeling low, start to drink (4)
I believe the answer is:
mood
'feeling' is the definition.
(mood is a kind of feeling)
'low start to drink' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'start to' suggests taking the first letters.
The initial letter of 'drink' is 'd'.
'moo'+'d'='MOOD'
(Other definitions for mood that I've seen before include "Frame of mind" , "Bad feeling" , "State of mind or feeling" , "Disposition" , "melancholy" .)