Occasionally parked on walk to find opening (8)
I believe the answer is:
preamble
'opening' is the definition.
(I've seen this in another clue)
'occasionally parked on walk' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'walk' becomes 'amble' (amble is a kind of walk).
The alternating letters of 'parked' are 'pre'.
'pre'+'amble'='PREAMBLE'
'to find' acts as a link.
(Other definitions for preamble that I've seen before include "Opening words" , "Explanatory foreword" , "Overture" , "Preface, opening statement" , "Foreword, introduction" .)