Crossword Genius

The test a heap may fail (3)

Ross

I believe the answer is:

mot

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

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

Can you help me to learn more?

(Other definitions for mot that I've seen before include "Clever remark" , "Witty words" , "French word for Tom" , "witty remark" , "type of test" .)

I've seen this clue in the Evening Standard.
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