Rudiments (6)

I think it could be either:

Which length description are you looking for?:

abcs

'rudiments' is the definition.
(synonyms)

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 Independent and The Wall Street Journal.

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