Channel swimmer crosses river (6)
I believe the answer is:
trench
'channel' is the definition.
(I've seen this in another clue)
'swimmer crosses river' is the wordplay.
I cannot really understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found 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 explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trench that I've seen before include "depression" , "Protective ditch" , "Ditch, as a fortification say" , "Fish; move on ice" , "A military ditch for soldiers" .)