Note it's on the level (4)
I believe the answer is:
flat
'level' is the definition.
(I know that level can be written as flat)
'note it's on the' is the wordplay.
'note' becomes 'f' (musical note).
'it's' becomes 'la' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'f'+'la'+'t'='FLAT'
Can you help me to learn more?
(Other definitions for flat that I've seen before include "rooms exactly" , "Personal residence" , "In music, below the correct pitch" , "like some races" , "Level; dull" .)