River horse, or cow, by the sound of it (5)
I believe the answer is:
rhone
'river' is the definition.
(Rhone River is an example)
'horse or cow by the sound of it' is the wordplay.
'horse or cow' becomes 'roan' (roan can mean an animal with certain colouring).
'by the sound of it' shows a homophone (sound like).
'roan' sounds like 'RHONE'.
(Other definitions for rhone that I've seen before include "River flowing from Alps to the French Med" , "French flower" , "wine-producing area" , "Current" , "River at Avignon and Arles" .)