School book (6)
I believe the answer is:
primer
'school book' 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 "Elementary textbook" , "Sealant coat" , "First coat - book" , "Introductory textbook or first coat of paint" , "device for detonating charge" .)