Adults only allowed in nick? (5)

I believe the answer is:

notch

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'nick?' is the definition.
('notch' can be a synonym of 'nick')

I don't understand the rest of the clue.

(Other definitions for notch that I've seen before include "Incision; indentation" , "Indentation - step up" , "Small V-shaped cut or indentation" , "Carved incision" , "V-shaped incision" .)

I've seen this clue in The Telegraph.

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