Sweating, out of breath, you need to sharpen up! (4)

I believe the answer is:

whet

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'sharpen up' is the definition.
'whet' can be an answer for 'sharpen' (whetting is a kind of sharpening). I'm unsure of the 'up' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for whet that I've seen before include "Hone" , "Sharpen the blade of (a tool/weapon)" , "Sharpen by rubbing" , "Make more acute" , "appetizer" .)

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