Sunny smile? (4)

I believe the answer is:

beam

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'smile?' is the definition.
(beam can mean to smile)

I cannot understand how the rest of the clue works.

(Other definitions for beam that I've seen before include "Ray; structural timber" , "Sturdy timber" , "Shine" , "Ray; piece of timber" , "Light - heavy timber - wide smile" .)

I've seen this clue in The Guardian and The Globe and Mail.

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