Bituminous substance (3)
'bituminous substance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tar that I've seen before include "Jack ..., British sailor" , "Sailor - laid on road" , "one may need to pay on boat" , "Art (anag.)" , "Kind of rat for sailor" .)
(Other definitions for asphalt that I've seen before include "Paving material" , "Bituminous substance used in building" , "Road-building material" , "Mixture of tar and gravel used in road surfaces" , "Dark natural pitch" .)