Ditch swimmer crossing river (6)
I believe the answer is:
trench
'ditch' is the definition.
('trench' can be a synonym of 'ditch')
'swimmer crossing river' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' and 'r' is found in the answer.
'swimmer' could be 'tench' (tench is a kind of fish) and 'tench' is found within the leftover 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 "A deep ditch, as dug for troops" , "Long, narrow excavation" , "Gutter" , "Drain" , "Long narrow ditch" .)