Mistake about one registered animal (7)
I believe the answer is:
giraffe
'animal' is the definition.
(I've seen this before)
'mistake about one registered' is the wordplay.
'mistake' becomes 'gaffe' (a gaffe is a mistake or error).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'registered' becomes 'R' (abbreviation, as in registered trademark).
'i'+'r'='ir'
'gaffe' going around 'ir' is 'GIRAFFE'.
(Other definitions for giraffe that I've seen before include "Sixteen, mostly neck" , "Long-necked browser" , "Animal once known as a camelopard" , "Long-necked mammal" , "one reaching highest places" .)