Black book central to special offer (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'book central to special offer' is the wordplay.
'book' becomes 'b'.
'central to' means one lot of letters goes inside another (letters are inserted into the centre of the word).
'special offer' becomes 'sale'.
'b' placed into 'sale' is 'SABLE'.
(Other definitions for sable that I've seen before include "Marten's fur - dark black" , "Black; antelope" , "Type of fur" , "Black (heraldry); fur" , "Bales of precious fur" .)