Introductory text (6)
'introductory text' 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" , "Initial coat of paint" , "introductory book" , "it might be used by one 7 [PAINTING]" , "First coat - book" .)
(Other definitions for preamble that I've seen before include "It starts" , "Explanatory foreword" , "Preliminary or introductory statement" , "Introductory explanation" , "Preface, opening statement" .)