A river fish caught in a moat (6)
I believe the answer is:
trench
'a moat' is the definition.
(thesaurus)
'a river fish' is the wordplay.
I cannot quite see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is located in the answer.
'fish' 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...
'caught in' is the link.
Can you help me to learn more?
(Other definitions for trench that I've seen before include "Drain" , "Deeply cut" , "Long, narrow excavation" , "Ditch, as a fortification say" , "depression" .)