Sort of numbers to learn first? (5)

I believe the answer is:

prime

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'first?' is the definition.
('prime' can be similar in meaning to 'first')

I can't explain the remainder of the clue.

(Other definitions for prime that I've seen before include "put in the picture" , "At least two" , "Prepare for use, say a pump" , "(Of number) divisible only by itself and one" , "Main; indivisible number" .)

I've seen this clue in the Evening Standard.

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