Low mood music (5)
I believe the answer is:
blues
'music' is the definition.
(blues is a kind of music)
'low mood' is the wordplay.
'low' becomes 'blue' (both can mean unhappy).
'mood' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'blue'+'s'='BLUES'
Can you help me to learn more?
(Other definitions for blues that I've seen before include "Type of American folksong" , "State of depression" , "Depression - music" , "Feeling of sadness or depression" , "Sort of music" .)