Surprise beginning to the half mile (7)
I believe the answer is:
startle
'surprise' is the definition.
(thesaurus)
'beginning to the half mile' is the wordplay.
'beginning' becomes 'start' ('start' can be a synonym of 'beginning').
'to the half' means to take half the letters.
'mile' cut in half is 'le'.
'start'+'le'='STARTLE'
(Other definitions for startle that I've seen before include "Cause to feel sudden alarm" , "Scare" , "Frighten suddenly" , "Surprise with fright" , "Alarm suddenly" .)