Low day for humour (4)
I believe the answer is:
mood
'humour' is the definition.
(I've seen this before)
'low day' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'day' becomes 'd'.
'moo'+'d'='MOOD'
'for' acts as a link.
(Other definitions for mood that I've seen before include "Frame of mind" , "Depression" , "Bad feeling" , "State of mind, humour" , "An atmosphere of doom" .)