Offender letting slip small secret (5)

I believe the answer is:

inner

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'secret' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for inner that I've seen before include "Ring round bulls-eye" , "Part of the bull" , "Nearer to the centre, deeper" , "Hidden" , "Ring next to the bullseye" .)

I've seen this clue in The Sunday Times.

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