Only three-letter scale note (3)
I believe the answer is:
sol
'only three-letter scale note' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sol that I've seen before include "Roman counterpart of Helios" , "Note; Peruvian currency" , "The Sun (or Campbell?)" , "The sun in Spain" , "Personification of the sun" .)