Ducks outside court as voting starts for book size (6)
I believe the answer is:
octavo
'book size' is the definition.
(I've seen this in another clue)
'ducks outside court as voting starts' is the wordplay.
'ducks' becomes 'oo' (duck means zero in cricket).
'outside' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'starts' suggests taking the first letters.
The initial letters of 'as voting' is 'av'.
'ct'+'av'='ctav'
'oo' going around 'ctav' is 'OCTAVO'.
'for' acts as a link.
(Other definitions for octavo that I've seen before include "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" , "An eight-leaved book or page" , "Common book size" , "Size of book page" .)