Introduction prior to stroll? (8)
I believe the answer is:
preamble
'introduction' is the definition.
(I know that preamble is a type of introduction)
'prior to stroll?' is the wordplay.
'prior to' becomes 'pre' (I've seen this in other clues).
'stroll?' becomes 'amble' ('amble' can be a synonym of 'stroll').
'pre'+'amble'='PREAMBLE'
(Other definitions for preamble that I've seen before include "Make introduction" , "Introductory part of document or law" , "the first thing one reads?" , "Explanatory foreword" , "It starts" .)