Understand, when the note is sounded (3)
I believe the answer is:
see
'understand' is the definition.
(I know that understand can be written as see)
'the note is sounded' is the wordplay.
'the note' becomes 'si' (I've seen this in other clues).
'is sounded' shows a homophone (sound like).
'si' sounds like 'SEE'.
'when' acts as a link.
(Other definitions for see that I've seen before include "Bishop's diocese" , "Consider; diocese" , "Observe the bishop's domain" , "date" , "Get < court < witness" .)