Brought about by half the cars being second-hand (6)
I believe the answer is:
caused
'brought about' is the definition.
(I've seen this before)
'by half the cars being second-hand' is the wordplay.
'by' says to put letters next to each other.
'half the' means to take half the letters.
'being second hand' becomes 'used' (second hand can mean previously owned or used).
'cars' with half the letters taken is 'ca'.
'ca'+'used' is 'CAUSED'.
(Other definitions for caused that I've seen before include "Produced as an effect" , "Brought about" , "Gave rise to" , "Made happen" .)