Tar on road turned grey (4)
I believe the answer is:
drab
'grey' is the definition.
(thesaurus)
'tar on road turned' is the wordplay.
'tar' becomes 'ab'.
'on' is a charade indicator (letters next to each other).
'road' becomes 'rd' (abbreviation).
'turned' shows that the letters should be reversed in order.
'rd' backwards is 'dr'.
'ab' after 'dr' is 'DRAB'.
(Other definitions for drab that I've seen before include "Dull and colourless" , "Gloomy" , "Grey or light brown cloth" , "Dreary and colourless" , "Dull and boring" .)