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Part
of
body
disappearing
as
one
stands
(3)
Part of body disappearing as one stands (3)
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The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue is a whimsical cryptic definition — Part of body disappearing as one stands does all the work here. Your LAP is very much present when you're sitting down, but the moment you get to your feet, it's gone. No wordplay to untangle; it's just a clever observation dressed up as a clue.