Secret contents (6)

I believe the answer is:

inside

I believe this clue is a double definition.

'secret' is the first definition.
('inside' can be a synonym of 'secret')

'contents' is the second definition.
(thesaurus)

(Other definitions for inside that I've seen before include "Imprisoned" , "Privileged" , "Not external, perhaps in prison" , "(Of information) from a confidential source" , "Stomach etc - in prison?" .)

I've seen this clue in The Telegraph.

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