Crossword Genius

Altogether the last three knockout rounds that spanned several years (9)

Ross

I believe the answer is:

seventies

'that spanned several years' is the definition.
(period of ten years)

I cannot understand how the rest of the clue works.

(Other definitions for seventies that I've seen before include "Decade of the Arms crisis, Vietnam and All Kinds of Everything" , "retirement years?" .)

I've seen this clue in The Times.
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