Slight quarrel about river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'slight quarrel about river' is the wordplay.
'slight quarrel' becomes 'spat' (I've seen this before).
'about' indicates putting letters inside.
'river' becomes 'r'.
'spat' placed around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "chap giving richer course material to the Dutch" , "Mackerel bait?" , "Small edible fish" , "'Set a . . . . . to catch a herring'" , "Small herring-type fish" .)