Animal's mistake eating fibre regularly (7)
I believe the answer is:
giraffe
'animal's' is the definition.
(thesaurus)
'mistake eating fibre regularly' is the wordplay.
'mistake' becomes 'gaffe' (gaffe is a kind of mistake**).
'eating' indicates putting letters inside.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'fibre' are 'ir'.
'gaffe' going around 'ir' is 'GIRAFFE'.
(Other definitions for giraffe that I've seen before include "Animal once known as a camelopard" , "Long - necked animal" , "Tallest beast" , "Tallest mammal" , "one reaching highest places" .)