Argument about river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'argument about river' is the wordplay.
'argument' becomes 'spat' (a spat is a disagreement or argument).
'about' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'spat' going around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Small fish, sardine, say" , "Jack --, non-fat eater" , "'Set a . . . . . to catch a whale'" , "Type of herring - small person or thing" , "Small herring-type fish" .)