Thick fog (3,4)
'thick fog' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pea-souper that I've seen before include "Thick smokey fog" , "Dense, dirty fog" , "Thick, dirty, yellowish fog" , "Dense yellow fog" .)
(Other definitions for pea soup that I've seen before include "Thick vegetable broth" , "Thick vegetable dish" , "very hazy situation" , "Hot dish (like a fog?)" , "Green potage" .)