Crossword Genius

One went into show business many, many years ago (5,3)

Ross

I believe the answer is:

stone age

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'many years ago' is the definition.

I cannot understand how the remainder of the clue works.

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(Other definitions for stone age that I've seen before include "Hard time" , "Earliest known period of human culture" , "Early period" , "before copper came in" , "Pre-historic era" .)

I've seen this clue in the Evening Standard.
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