Fundamentals (4)

I think it could be either:

Which length description are you looking for?:

6 4*

'fundamentals' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for abcs that I've seen before include "Know these to be well versed - alphabetically?" , "12 [THINGS EASY]" , "guides ..." , "Know these letters to be well informed" .)

I've seen this clue in The New York Times, the LA Times and the Universal.

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