Domestic military information to protect Royal Navy area (8)

I believe the answer is:

internal

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'domestic' is the definition.
(similar in meaning)

'military information to protect royal navy area' is the wordplay.
'military information' becomes 'intel' (I've seen this before. I am not sure about the 'military' bit.).
'to protect' indicates putting letters inside (inserted letters are held or protected).
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'area' becomes 'a' (maths abbreviation).
'rn'+'a'='rna'
'intel' enclosing 'rna' is 'INTERNAL'.

Can you help me to learn more?

(Other definitions for internal that I've seen before include "Inside the body" , "Within the body" , "Located inward" , "Domestic" , "Interior" .)

I've seen this clue in The Telegraph.

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