Crossword Genius

Slag off spurious register plate belonging to the past (9)

Ross

I believe the answer is:

preterite

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'the past' is the definition.
(preterite is a kind of past)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for preterite that I've seen before include "tense" , "past form" .)

I've seen this clue in the Sydney Morning Herald.
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