An argument about River Dart (5)
I believe the answer is:
arrow
'dart' is the definition.
(I've seen this before)
'an argument about river' is the wordplay.
'an' becomes 'a'.
'argument' becomes 'row' (to row is to argue).
'about' says to put letters next to each other.
'river' becomes 'r' (abbreviation).
'row' put after 'r' is 'rrow'.
'a'+'rrow'='ARROW'
(Other definitions for arrow that I've seen before include "Move swiftly" , "Early guided missile" , "Archery projectile" , "Something pointing" , "Type of projectile" .)