It's a very good headlight (4)
I believe the answer is:
halo
'headlight' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Aura representing saintliness" , "Corona" , "Circle of light around the moon" , "Hagiological aura featured in Christian art" , "Sign of sainthood" .)