Regrets deposit gone astray for Royal Mail service? (10,4)

I believe the answer is:

registered post

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'royal mail service?' is the definition.
The definition suggests a singular noun which matches the answer.

'regrets deposit gone astray' is the wordplay.
'gone astray' indicates anagramming the letters (I've seen this in other clues).
'regrets'+'deposit'='regretsdeposit'
'regretsdeposit' anagrammed gives 'REGISTERED POST'.

'for' is the link.

Can you help me to learn more?

(Other definitions for registered post that I've seen before include "Type of mail" , "Special form of delivery" , "sign for this?" , "Specially secure delivery" , "important letters? - with 28a" .)

I've seen this clue in The Telegraph.

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