*Road goo (4)
(Other definitions for tsar that I've seen before include "specific authority" , "Male monarch" , "Russian monarch" , "Absolute ruler" , "Star for a Russian emperor" .)
'road goo' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tar that I've seen before include "Seaman; pitch" , "Sailor (colloq.)" , "Familiar sailor - what a rat" , "prepare for feathers?" , "Dark viscous stuff" .)