Piece of cake, al fresco treat (6)

I believe the answer is:

picnic

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'piece of cake' is the definition.
(both can mean something easy)

I can't explain the rest of the clue.

(Other definitions for picnic that I've seen before include "Do without" , "Meal outdoors" , "external courses?" , "Take a meal outside" , "Outdoor repast" .)

I've seen this clue in The Times.

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