Initially important guessing game (1-3)

I believe the answer is:

i-spy

I'm a little stuck... Click here to teach me more about this clue!

'guessing game' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for i-spy that I've seen before include "Child's game: ''. ... with my little eye'" , "Children's observational game" , "Children's guessing game" , ". ... with my little eye (children's game)" , "Children's alphabet game" .)

I've seen this clue in The Guardian.

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