Tall animal's error crossing one river (7)
I believe the answer is:
giraffe
'tall animal's' is the definition.
(giraffes are tall animals)
'error crossing one river' is the wordplay.
'error' becomes 'gaffe' (gaffe is a kind of error).
'crossing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'river' becomes 'r' (abbreviation for river).
'i'+'r'='ir'
'gaffe' going around 'ir' is 'GIRAFFE'.
(Other definitions for giraffe that I've seen before include "Tallest beast" , "Sixteen, mostly neck" , "The largest ruminant" , "Long - necked animal" , "one reaching highest places" .)