Planetary lighting centre (3)
I believe the answer is:
sun
'planetary lighting centre' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for sun that I've seen before include "Rag" , "Helios, in Greek myth" , "Get a tan" , "It's sol in Latin" , "Nearest star to Earth" .)