Tiff involving river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'tiff involving river' is the wordplay.
'tiff' becomes 'spat' ('spat' can be a synonym of 'tiff').
'involving' indicates putting letters inside.
'river' becomes 'r'.
'spat' placed around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "'Set a . . . . . to catch a herring'" , "Traps (anag.)" , "Strap up the small fish" , "'Set a . . . . . to catch a whale'" , "little fish" .)