Crossword Genius

When to tell all? (6)

Ross

I believe the answer is:

census

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'tell all?' is the definition.
'census' can be an answer for 'tell' (thesaurus). I am unsure of the 'all?' bit.

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

Can you help me to learn more?

(Other definitions for census that I've seen before include "Periodic count as of population" , "Official survey" , "Official body count?" , "Periodical population count" , "Head count" .)

I've seen this clue in The Globe and Mail.
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