Animal in book — a piglet? (5)
I believe the answer is:
'animal' is the definition.
'in book a piglet?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OKAPI' is hidden within 'book a piglet'.
(Other definitions for okapi that I've seen before include "browser" , "African mammal related to giraffe" , "Animal, a sort of zebra/giraffe cross" , "One of the giraffe family" , "Ruminant mammal of Africa" .)