Animal hit with book (4)
I believe the answer is:
lamb
'animal' is the definition.
(lamb is a kind of animal)
'hit with book' is the wordplay.
'hit' becomes 'lam' (to 'lam' means to hit hard).
'with' means one lot of letters go next to another.
'book' becomes 'b'.
'lam'+'b'='LAMB'
(Other definitions for lamb that I've seen before include "Youngster" , "Young farm animal" , "Writer" , "Exemplar of innocence" , "Skipper" .)