Showing rashness, I must abandon car at a big town (8)
I believe the answer is:
audacity
'showing rashness' is the definition.
(audacity can mean rashness or recklessness)
'i must abandon car at a big town' is the wordplay.
'i must abandon car' becomes 'aud' (I can't justify this - if you can you should believe this answer much more).
'at' means one lot of letters go next to another.
'big town' becomes 'city' (I've seen this before).
'aud'+'a'+'city'='AUDACITY'
(Other definitions for audacity that I've seen before include "Boldness or cheeky daring" , "Courage" , "Brass neck, boldness" , "Effrontery" , "Cheek, nerve" .)