Serpent to stop at the surface (7)
I believe the answer is:
asphalt
'the surface' is the definition.
(I have seen 'surface of path' mean 'asphalt' so perhaps 'surface' could also mean 'asphalt')
'serpent to stop' is the wordplay.
'serpent' becomes 'asp' (asp is a kind of serpent).
'to stop' becomes 'halt' (halt can mean to stop).
'asp'+'halt'='ASPHALT'
'at' acts as a link.
(Other definitions for asphalt that I've seen before include "petroleum residue" , "Bituminous material for roofing, paths etc" , "Bituminous road dressing" , "Roofers use this" , "Bituminous substance used for roofs and roads" .)