Authority impounds contents of tube tunnel (6)

I believe the answer is:

subway

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'tunnel' is the definition.
(I know that Subway is a type of tunnel)

'authority impounds contents of tube' is the wordplay.
'authority' becomes 'sway' (I can't explain this - if you can you should believe this answer much more).
'impounds' is an insertion indicator.
'contents of' means to look at the middle letters (the letters on the inside of the word).
The middle letters of 'tube' are 'ub'.
'sway' enclosing 'ub' is 'SUBWAY'.

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(Other definitions for subway that I've seen before include "New York underground" , "crossing point" , "American tube" , "Underground rail" , "New York's "tube"" .)

I've seen this clue in The Daily Mail.

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