Crossword Genius

Type date, line going over part of index (3,4)

Ross

I believe the answer is:

key word

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'part of index' is the definition.
('key word' is part of 'index')

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for key word that I've seen before include "Descriptive term or clue" , "Significant term" .)

I've seen this clue in The Guardian.
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