Petty quarrel about river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'petty quarrel about river' is the wordplay.
'petty quarrel' becomes 'spat' (I've seen this before).
'about' is an insertion indicator.
'river' becomes 'r'.
'spat' placed around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Mackerel bait?" , "Parts (anag.)" , "chap giving richer course material to the Dutch" , "Jack, perhaps" , "Traps (anag.)" .)