Vehicle of retrograde art, to many (4)
I believe the answer is:
tram
'vehicle' is the definition.
(tram is a kind of vehicle)
'retrograde art to many' is the wordplay.
'retrograde' shows that the letters should be reversed in order.
'to many' becomes 'm' (Roman numeral for a thousand).
'art' backwards is 'tra'.
'tra'+'m'='TRAM'
'of' acts as a link.
(Other definitions for tram that I've seen before include "Urban transport" , "It runs on rails" , "Rail vehicle" , "Take it to the mart" , "Electrically driven transport" .)