Phaetons replaced single-seater carriage (8)
I believe the answer is:
stanhope
'single-seater carriage' is the definition.
'phaetons replaced' is the wordplay.
'replaced' indicates anagramming the letters.
'phaetons' is an anagram of 'STANHOPE'.
(Other definitions for stanhope that I've seen before include "Light horse-drawn carriage; Nineteen across (anag.)" , "It may have only two wheels" , "Phaetons can be turned into a light open one-seater carriage" , "vehicle" , "carriage that has seat only for one" .)