Row about river fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'row about river' is the wordplay.
'row' becomes 'spat' (a spat is a disagreement or argument).
'about' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'spat' placed around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "little fish" , "Small sea fish" , "Small herring-type fish" , "Small fish (used to catch mackerel?)" , "Young herring" .)