Brought up Talk Radio’s secret (4)

I believe the answer is:

dark

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'secret' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for dark that I've seen before include "Nightfall; without light" , "Shade of Oxford blue" , "Black, perhaps - night" , "Swarthy" , "Devoid of brightness" .)

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