Minor argument about river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'minor argument about river' is the wordplay.
'minor argument' becomes 'spat' (I've seen this before).
'about' means one lot of letters goes inside another.
'river' becomes 'r'.
'spat' enclosing 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "'Set a . . . . . to catch a herring'" , "Small herring-like fish" , "Jack, perhaps" , "Traps (anag.)" , "Herring's smaller relative" .)