Where cars race about madly, dashing at intervals (8)
I believe the answer is:
autobahn
'where cars race' is the definition.
(an autobahn is a German motorway)
'about madly dashing at intervals' is the wordplay.
'madly' indicates an anagram.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
The alternating letters of 'dashing' are 'ahn'.
'about' is an anagram of 'autob'.
'autob'+'ahn'='AUTOBAHN'
(Other definitions for autobahn that I've seen before include "expressway" , "German or Austrian motorway" , "German lanes" , "Great way" , "way abroad" .)