Animal's mistake, crossing one river (7)
I believe the answer is:
giraffe
'animal's' is the definition.
(thesaurus)
'mistake crossing one river' is the wordplay.
'mistake' becomes 'gaffe' (gaffe is a kind of mistake**).
'crossing' means one lot of letters goes inside another.
'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 "Fire fag at the animal" , "that's beastly" , "A long-legged creature" , "Large African mammal" , "The tallest animal with long neck" .)