Petty quarrel involving river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'petty quarrel involving river' is the wordplay.
'petty quarrel' becomes 'spat' (I've seen this before).
'involving' means one lot of letters goes inside another.
'river' becomes 'r'.
'spat' placed around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Herring of the east North Atlantic" , "Small sea fish" , "Strap up the small fish" , "chap giving richer course material to the Dutch" , "Small herring-like fish" .)