Thick, like fog (5)
I believe the answer is:
dense
'thick like fog' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dense that I've seen before include "Thick, impenetrable" , "Compressed - obtuse" , "Thick and tightly-packed" , "Close knit" , "Needs (anag) -- thick" .)