Animal makes blunder crossing one river (7)
I believe the answer is:
giraffe
'animal' is the definition.
(thesaurus)
'blunder crossing one river' is the wordplay.
'blunder' becomes 'gaffe' (gaffe is a kind of blunder**).
'crossing' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'river' becomes 'r'.
'i'+'r'='ir'
'gaffe' going around 'ir' is 'GIRAFFE'.
'makes' is the link.
(Other definitions for giraffe that I've seen before include "Long-necked animal" , "Large African mammal" , "Long-necked ruminant" , "Tallest living quadruped" , "The animal might fear fig" .)