Understood sensitivity surrounding self-referential letter (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 can't explain the rest of the clue.

(Other definitions for tacit that I've seen before include "Respect, admiration" , "without speaking" , "Wordless" , "Implicit, unspoken" , "Silently understood" .)

I've seen this clue in The Times.

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