Note how sunny he is! (3)
I believe the answer is:
sol
This clue might be a double definition.
'note' is the first definition.
(musical note as in do-re-mi-fa-sol-la-ti)
'sunny he is' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'how' is the link.
Can you help me to learn more?
(Other definitions for sol that I've seen before include "The sun in Spain" , "Roman counterpart of Helios" , "Spanish and poetic sun" , "nearby star" , "Fifth note of scale" .)