Begin official march outside (7)
I believe the answer is:
preface
'begin' is the definition.
(thesaurus)
'official march outside' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'march' becomes 'pace' (I've seen this in other clues).
'outside' indicates putting letters inside.
'ref' put within 'pace' is 'PREFACE'.
(Other definitions for preface that I've seen before include "Foreword" , "Introductory text" , "Opening words" , "Introduction (to a book)" , "Book's introduction" .)