Perhaps preacher’s examination marginalised by lack of clarity? (8)
I believe the answer is:
moralist
'perhaps preacher's' is the definition.
(a preacher is someone who moralises)
'examination marginalised by lack of clarity?' is the wordplay.
'examination' becomes 'oral' (oral is a kind of examination).
'marginalised by' indicates putting letters inside (outer letters form the inserted letters' margins).
'lack of clarity?' becomes 'mist' (mist can mean vagueness or lack of clarity).
'oral' inserted within 'mist' is 'MORALIST'.
(Other definitions for moralist that I've seen before include "preacher" , "Principled type" , "Ail storm (anag)" , "Person of principle" , "Ethical figure" .)