Crossword Genius

Channel swimmer crossing river (6)

Ross

I believe the answer is:

trench

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'channel' is the definition.
(thesaurus)

'swimmer crossing river' is the wordplay.
I cannot really understand how this works, but
'river' could be 'r' and 'r' is located in the answer.
'swimmer' could be 'tench' (tench is a kind of fish) and 'tench' is located in the remaining letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for trench that I've seen before include "Deeply cut" , "Long narrow ditch" , "A military ditch for soldiers" , "depression" , "Gutter" .)

I've seen this clue in The Telegraph.
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