Be brave and exercise (4-2)

I believe the answer is:

chin-up

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

I cannot understand how the remainder of the clue works.

(Other definitions for chin-up that I've seen before include "Don't despair" , "Be of good cheer - elevate mandible" , "Remain cheerful - mandible aloft" , "Remain cheerful, elevate mandible" , "Keep cheerful! (colloq.)" .)

I've seen this clue in The Irish Times.

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