Badly built rental vehicle that's rapid (6,5)
I believe the answer is:
bullet train
'vehicle that's rapid' is the definition.
(fast train)
'badly built rental' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'built'+'rental'='builtrental'
'builtrental' with letters rearranged gives 'BULLET TRAIN'.
(Other definitions for bullet train that I've seen before include "Get to work on it fast!" , "it travels quickly" , "Japanese high-speed transport" .)