Bookmaker's size deceives better
I believe the answer is:
fools cap
'bookmaker's size' is the definition.
'fools cap' can be an answer for 'size' (I have seen 'Size of paper ' mean 'fools cap' so perhaps 'size' could also mean 'fools cap'). I am unsure of the 'bookmaker's' bit.
'deceives better' is the wordplay.
'deceives' becomes 'fools' (fooling is a kind of deceiving).
'better' becomes 'cap' (I've seen this before).
'fools'+'cap'='FOOLS CAP'
Can you help me to learn more?
(Other definitions for fools cap that I've seen before include "Cop's foal (anag)" , "About 330 x 200 (or 400) mm size of paper" , "Old writing paper size -- cops loaf (anag)" , "Writing paper like dunce's headgear" , "Paper or book size (from its watermark)" .)