A boy's light (4)

I believe the answer is:

rays

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'light' is the definition.
(I know this)

I cannot understand how the rest of the clue works.

(Other definitions for rays that I've seen before include "Straight lines extending from a point" , "Narrow beams of light" , "Columns of light or heat" , "Fish with wing-shaped fins and long tails" , "They swim" .)

I've seen this clue in the Evening Standard.

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