Crossword Genius

River culvert, perhaps avoiding the first gutter (6)

Ross

I believe the answer is:

runnel

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'gutter' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for runnel that I've seen before include "Brook; gutter" , "Rivulet" , "A small stream or brook" , "Small stream or gutter" .)

I've seen this clue in The Independent.
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