Fish around river channel (6)

I believe the answer is:

trench

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This could be a double definition.

'fish around river' is the first definition.
'trench' can be an answer for 'fish'. I am unsure of the 'around river' bit.

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

Can you help me to learn more?

(Other definitions for trench that I've seen before include "A ditch dug by soldiers" , "Ditch dug as fortification" , "at the front" , "A military ditch for soldiers" , "Military cut in the earth" .)

I've seen this clue in The Sun.

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