Fundamental right to a textbook (6)

I believe the answer is:

primer

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'to a textbook' is the definition.
(primer is a kind of textbook)

I don't understand the remainder of the clue.

(Other definitions for primer that I've seen before include "Basic book; undercoat" , "Elementary text book" , "Solitary" , "introductory book" , "it might be used by one 7 [PAINTING]" .)

I've seen this clue in The Mirror.

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