A thousand ducks quacked excitedly, having united north of the Channel (8)
I believe the answer is:
aqueduct
'the channel' is the definition.
(aqueduct is a kind of channel)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for aqueduct that I've seen before include "Raised water conduit" , "High water channel" , "Conduit to carry water over a valley" , "Carrier of water over valley" , "Artificial channel carrying water across country" .)