Show by your expression, when you enter (8)

I believe the answer is:

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'enter' is the definition.
(I know that register is a type of enter)

I can't explain the remainder of the clue.

(Other definitions for register that I've seen before include "a manner of speaking" , "part of our paper" , "To express" , "(Enrol in) list" , "Log" .)

I've seen this clue in the Evening Standard.

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