Introduction before gentle walk (8)
I believe the answer is:
preamble
'introduction' is the definition.
(I know that preamble is a type of introduction)
'before gentle walk' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'gentle walk' becomes 'amble' (an amble is a relaxed walk).
'pre'+'amble'='PREAMBLE'
(Other definitions for preamble that I've seen before include "Introductory or opening statement" , "Foreword, introduction" , "Overture" , "Introductory explanation" , "Opening words" .)