Salt put on waterproof surface of road? (6)
I believe the answer is:
tarmac
'surface of road?' is the definition.
(tarmacadam is a road surfacing material)
'salt put on waterproof' is the wordplay.
'salt' becomes 'tar' (both can informally mean a sailor).
'put on' means one lot of letters go next to another.
'waterproof' becomes 'mac' (mac is a kind of waterproof).
'tar'+'mac'='TARMAC'
(Other definitions for tarmac that I've seen before include "Road surface material of tar and stone" , "Trade name for road surfacing material" , "Planes taxi on it" , "Surfaced area as for planes to take off and land" , "Road-surfacing substance" .)