One that's clearly understood public order (4,4)
I believe the answer is:
open book
'one that's clearly understood' is the definition.
(an easily understood person is an open book)
'public order' is the wordplay.
'public' becomes 'open' (associated in meaning).
'order' becomes 'book' (order can mean to reserve or book something).
'open'+'book'='OPEN BOOK'
(Other definitions for open book that I've seen before include "Person easily understood" , "Face etc that can be easily read" , "Something that can be easily interpreted" .)