River culvert, perhaps avoiding the first gutter (6)
I believe the answer is:
runnel
'gutter' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
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" .)