Go out of this world, say, in zero visibility (3-6)
I believe the answer is:
pea-souper
'zero visibility' is the definition.
The definition suggests a singular noun which matches the answer.
'go out of this world say' is the wordplay.
'go' becomes 'pea' (I am not sure about this - if you are sure you should believe this answer much more).
'out of this world' becomes 'super' (out of this world can mean excellent or super).
'say' shows a homophone (sound like).
'super' sounds like 'souper'.
'pea'+'souper'='PEA-SOUPER'
'in' acts as a link.
(Other definitions for pea-souper that I've seen before include "Dense yellow fog" , "Dense, dirty fog" , "Thick smokey fog" , "Thick, dirty, yellowish fog" .)