Wild animals making mistakes crossing one river (8)
I believe the answer is:
giraffes
'wild animals' is the definition.
(giraffes are wild animals)
'mistakes crossing one river' is the wordplay.
'mistakes' becomes 'gaffes' (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'
'gaffes' going around 'ir' is 'GIRAFFES'.
'making' is the link.
(Other definitions for giraffes that I've seen before include "Tallest quadrupeds" , "Long-necked animals" .)