A letter to regret, being the unvarnished facts (4)

I believe the answer is:

true

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'a letter to regret being the unvarnished facts' is the definition.
I can't judge whether this defines the answer.

'a letter to regret' is the wordplay.
'a letter' becomes 't' ().
'to regret' becomes 'rue' (to rue something is to regret it).
't'+'rue'='TRUE'

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(Other definitions for true that I've seen before include "Dependable -- loyal" , "Bona fide" , "Right - honourable" , "Genuine, real" , "Right; genuine" .)

I've seen this clue in the Evening Standard.

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