G, in C (3)
I believe the answer is:
sol
'g in c' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sol that I've seen before include "Note - colloid - the Sun" , "Roman counterpart of Helios" , "Musical note" , "The sun in 12 Across" , "Personification of the sun" .)