River grass rising to surface (6)
I believe the answer is:
tarmac
'surface' is the definition.
(tarmacing is a kind of surfacing)
'river grass rising' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'grass' becomes 'rat' (both can mean a snitch or informer).
'rising' shows that the letters should be reversed in order (going up in a down clue).
'cam'+'rat'='camrat'
'camrat' reversed gives 'TARMAC'.
'to' is the link.
(Other definitions for tarmac that I've seen before include "something for the road" , "Airport runway surface" , "black one" , "Road paving material" , "Topping" .)