Low-down humour (4)
I believe the answer is:
mood
'humour' is the definition.
(I've seen this before)
'low-down' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'down' becomes 'd'.
'moo'+'d'='MOOD'
(Other definitions for mood that I've seen before include "Depression" , "Temporary state of mind" , "Bad feeling" , "Feeling of doom" , "sullenness may be indicative" .)