Upmarket transport (4)
I believe the answer is:
tram
'transport' is the definition.
(tram is a kind of transport)
'up-market' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'market' becomes 'mart' (synonyms).
'mart' back-to-front is 'TRAM'.
(Other definitions for tram that I've seen before include "Bus on rails" , "Passenger vehicle on rails" , "An early form of Luas in Dublin transport" , "Urban transport" , "Old city transport, ram on rails through streets" .)