Crossword Genius

It's a very good headlight (4)

Ross

I believe the answer is:

halo

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'headlight' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't 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" .)

I've seen this clue in The Irish News.
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