Bridges over valleys (8)
I believe the answer is:
viaducts
'bridges over valleys' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for viaducts that I've seen before include "Bridges to carry roads and railways over valleys" , "Arches supporting roads and railways over valleys" , "Arched routes over valleys" , "Bridges with arches spanning valleys" .)