Stellar radio source (6)
I believe the answer is:
quasar
'stellar radio source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for quasar that I've seen before include "Heavenly radiator" , "Distant, mystery sky object" , "Starlike source of radiation" , "Very luminous and very distant astronomical object" , "Source of radiation in space" .)