Rest or exercise can help keep you so (4)

I believe the answer is:

lean

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'rest' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for lean that I've seen before include "Slender" , "without success" , "Wiry" , "'Tend, incline (4)'" , "Unproductive" .)

I've seen this clue in the Evening Standard.

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