Elementary textbook (6)
I believe the answer is:
primer
'elementary textbook' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for primer that I've seen before include "explosive device" , "Introductory textbook or first coat of paint" , "Preparatory coat of paint" , "Basic book; undercoat" , "First coat - book" .)