One's not at all clear or sure -- a pope should clarify (3-6)
I believe the answer is:
pea-souper
'one's not at all clear' is the definition.
(term for a thick fog)
'sure a pope should clarify' is the wordplay.
'should clarify' is an anagram indicator (letters become clearer or more organised).
'sure'+'a'+'pope'='sureapope'
'sureapope' with letters rearranged gives 'PEA-SOUPER'.
'or' is the link.
(Other definitions for pea-souper that I've seen before include "Thick, dirty, yellowish fog" , "Dense, dirty fog" , "Dense yellow fog" , "Thick smokey fog" .)