Road gunk (3)
I believe the answer is:
tar
'road gunk' 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 "Mariner, old salt" , "Thick flammable liquid" , "Cigarette substance" , "Jack ..., British sailor" , "Sailor and dressing for roads" .)