Writer's hit book (4)
I believe the answer is:
lamb
'writer's' is the definition.
(I've seen this before)
'hit book' is the wordplay.
'hit' becomes 'lam' (to 'lam' means to hit hard).
'book' becomes 'b' (abbreviation for book).
'lam'+'b'='LAMB'
(Other definitions for lamb that I've seen before include "Skipper" , "Young sheep" , "Young creature" , "English essayist" , "Agnus Dei - . . . . of God" .)