Reportedly where Colossus overlooked harbour raids (7)
I believe the answer is:
inroads
'raids' is the definition.
(inroad can mean a raid or attack)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for inroads that I've seen before include "Encroachments" , "Progress" , "Sad iron (anag.)" , "Hostile incursions" , "Hostile attacks" .)