Deposit one left in road (4)
I believe the answer is:
silt
'deposit' is the definition.
(thesaurus)
'one left in road' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'road' becomes 'st'.
'i'+'l'='il'
'il' going into 'st' is 'SILT'.
(Other definitions for silt that I've seen before include "Mud deposited in a river or lake" , "Sediment deposited in channel or harbour" , "Sediment in a stream" , "Sedimentary material" , "Fine sediment (often deposited by water)" .)