Beginning to etch interior (5)

I believe the answer is:

onset

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'beginning' is the definition.
(onset is a kind of beginning)

I cannot understand how the remainder of the clue works.

(Other definitions for onset that I've seen before include "Beginning (of something often unpleasant)" , "Storming" , "Start, beginning" , "Attack using stone" , "Start - attack" .)

I've seen this clue in the Sydney Morning Herald.

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