Freshwater fish crossing river channel (6)
I believe the answer is:
trench
'river channel' is the definition.
'trench' can be an answer for 'channel' (thesaurus). I am not certain of the 'river' bit.
'freshwater fish crossing' is the wordplay.
'freshwater' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'fish' becomes 'tench' (tench is a kind of fish).
'crossing' means one lot of letters goes inside another.
'r' inserted within 'tench' is 'TRENCH'.
Can you help me to learn more?
(Other definitions for trench that I've seen before include "A deep ditch, as dug for troops" , "A ditch dug by soldiers" , "at the front" , "Ditch, as a fortification say" , "Military cut in the earth" .)