It's mad to be so abstemious - take a bit of this delicious dessert. (6)

I believe the answer is:

mousse

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'dessert' is the definition.
(I know that mousse is a type of dessert)

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

Can you help me to learn more?

(Other definitions for mousse that I've seen before include "Creamy substance used for styling hair" , "Foamy hair preparation" , "Gel" , "Whipped sweet" , "Aerosol foam used in hair styling" .)

I've seen this clue in The Irish Times.

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