One wheels in books for baker's agent (6)
I believe the answer is:
bicarb
'baker's agent' is the definition.
(leavening agent used in baking)
'one wheels in books' is the wordplay.
'one' becomes 'i' (Roman numeral).
'wheels' becomes 'car' (car wheel is a kind of wheel).
'in' means one lot of letters goes inside another.
'books' becomes 'bb' (two abbreviations for book).
'i'+'car'='icar'
'icar' inserted within 'bb' is 'BICARB'.
'for' is the link.
(Other definitions for bicarb that I've seen before include "It helps digestion" , "What helps raise" , "Baking soda (abbr)" , "source of fizz?" , "baking aid" .)