Well-proportioned-sounding old cab (6)
I believe the answer is:
hansom
'old cab' is the definition.
'well-proportioned-sounding' is the wordplay.
'well-proportioned' becomes 'handsome' (I've seen this before).
'sounding' indicates a 'sounds like' (homophone) clue.
'handsome' sounds like 'HANSOM'.
(Other definitions for hansom that I've seen before include "Two-wheeled one-horse carriage" , "Old horse cab sounds attractive" , "Type of cab" , "Victorian cab" , "Vehicle with back-seat driver" .)