Paint a textbook (6)
I believe the answer is:
primer
I believe this clue is a double definition.
'paint' is the first definition.
(I know that primer is a type of coat of paint)
'a textbook' is the second definition.
(I know that primer is a type of textbook)
(Other definitions for primer that I've seen before include "Reading book" , "First coat - book" , "Elementary text book" , "introductory book" , "He charges" .)