Mistake to capture one river animal (7)
I believe the answer is:
giraffe
'animal' is the definition.
(thesaurus)
'mistake to capture one river' is the wordplay.
'mistake' becomes 'gaffe' (gaffe is a kind of mistake**).
'to capture' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'river' becomes 'r'.
'i'+'r'='ir'
'gaffe' going around 'ir' is 'GIRAFFE'.
(Other definitions for giraffe that I've seen before include "Long-necked mammal" , "one reaching highest places" , "Long-necked animal" , "Animal once known as a camelopard" , "Tallest beast" .)