A note, true in each case (5)
I believe the answer is:
every
'a note true in each case' is the definition.
I can't judge whether this defines the answer.
'note true in each case' is the wordplay.
'note' becomes 'e' (musical note).
'true in each case' becomes 'very' (I can't justify this - if you can you should give a lot more credence to this answer).
'e'+'very'='EVERY'
Can you help me to learn more?
(Other definitions for every that I've seen before include "Each one without exception" , "Each of a number" , "The best possible" , "Each (one of a set)" , "Each single (item)" .)