Take mild exercise that may make one taller? (7,4,4)

I believe the answer is:

stretch ones legs

'take mild exercise' is the definition.

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

(Other definitions for stretch ones legs that I've seen before include "Go for a short walk" , "stroll" , "as Procrustes might do" , "Take a short walk" , "Take a walk for exercise" .)

I've seen this clue in The Times.

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