He sounds a note (3)
I believe the answer is:
ray
'he' is the definition.
'sounds a note' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'a note' becomes 're'.
're' is a homophone of 'RAY'.
(Other definitions for ray that I've seen before include "Narrow beam of light" , "bit of sunshine" , "may be electric" , "Flatfish or sunbeam" , "Column of light on the fish?" .)