A snake has to stop on the road surface (7)
I believe the answer is:
asphalt
'the road surface' is the definition.
(I've seen this in another clue)
'a snake has to stop' is the wordplay.
'a snake' becomes 'asp' (asp is a kind of snake).
'has' means one lot of letters go next to another.
'to stop' becomes 'halt' (halt can mean to stop).
'asp'+'halt'='ASPHALT'
'on' is the link.
(Other definitions for asphalt that I've seen before include "Bituminous material for roofing, paths etc" , "petroleum residue" , "Mixture of tar and gravel for paving" , "that's found on the way" , "Paving material" .)