Test passed by only fifty per cent of motors? (3)

I believe the answer is:

mot

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'test' is the definition.
(as in an MOT test for a vehicle)

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

(Other definitions for mot that I've seen before include "Word in France, bon perhaps" , "Wisecrack (for older cars!)" , "Clever remark" , "Witty words" , "French word for Tom" .)

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