Conduit that carries water over a valley (8)
I believe the answer is:
aqueduct
'conduit that carries water over a valley' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aqueduct that I've seen before include "Carrier of water over valley" , "Raised water conduit" , "Conduit like a bridge for carrying water" , "Water-carrying bridge" , "High water channel" .)