Note star is downcast without this (5)
I believe the answer is:
minus
'downcast without this' is the definition.
The definition suggests a singular noun but the answer is not.
'note star' is the wordplay.
'note' becomes 'mi' (as in do-re-mi).
'star' becomes 'nus' (I can't explain this - if you can you should believe this answer much more).
'mi'+'nus'='MINUS'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for minus that I've seen before include "Lacking; below zero" , "Deprived of" , "Take-away (sign)" , "Reduced by the subtraction of - thus?" , "'Take away' sign" .)