Adjacent notes on edge (6)
I believe the answer is:
beside
'adjacent' is the definition.
(I've seen this before)
'notes on edge' is the wordplay.
'notes' becomes 'be'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'edge' becomes 'side' (edge can mean the side of something).
'be'+'side'='BESIDE'
(Other definitions for beside that I've seen before include "Next door to" , "- - the point" , "Next to - compared with" , "Neighbours" , "Adjacent to" .)