Spread in book, say (6)
I believe the answer is:
butter
'spread' is the definition.
(I've seen this before)
'book say' is the wordplay.
'book' becomes 'b'.
'say' becomes 'utter' (I've seen this before).
'b'+'utter'='BUTTER'
'in' is the link.
(Other definitions for butter that I've seen before include "Spread derived from cream" , "Edible fat" , "(and 4) Kind of toffee" , "goat?" , "If it wouldn't melt in your mouth, you're innocent" .)