Exercise allows one to delay retiring (3,2)

I believe the answer is:

sit up

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'exercise' is the definition.
(I know that sit-up is a type of stomach exercise)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for sit up that I've seen before include "crunch" , "Don't slouch" , "Exercise done on the floor" , "Exercise move" , "Become suddenly alert" .)

I've seen this clue in The Guardian.

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