'How do you spell "shy"?' That's pretty light stuff (6)

I believe the answer is:

shandy

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'light stuff' is the definition.
(beer diluted with lemonade)

I don't understand how the remainder of the clue works.

(Other definitions for shandy that I've seen before include "Beer/lemonade mixture" , "Light beer" , "Beer-lemonade mix" , "Drink made of beer and lemonade" , "Alcohol" .)

I've seen this clue in The Everyman puzzle in The Observer.

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