Understood that many secondary pupils start progressively (5)

I believe the answer is:

tacit

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'understood' is the definition.
(I know that understood can be written as tacit)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tacit that I've seen before include "Implied but not directly expressed" , "Understood without being directly expressed" , "Unspoken but implied" , "Unspoken but understood" , "Silent but implied" .)

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

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