Mistake involves one registered animal (7)
I believe the answer is:
giraffe
'animal' is the definition.
(I know that giraffe is a type of animal)
'mistake involves one registered' is the wordplay.
'mistake' becomes 'gaffe' (a gaffe is a mistake or error).
'involves' 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 "The largest ruminant" , "Tall quadruped" , "Fire fag at the animal" , "A long-legged creature" , "Long-necked animal" .)