Quietly use a rope to get around in thick fog (3-6)
I believe the answer is:
pea-souper
'thick fog' is the definition.
(I've seen this before)
'quietly use a rope to get around' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'to get around' indicates anagramming the letters (letters move around).
'use'+'a'+'rope'='usearope'
'usearope' with letters rearranged gives 'easouper'.
'p'+'easouper'='PEA-SOUPER'
'in' is the link.
(Other definitions for pea-souper that I've seen before include "Dense, dirty fog" , "Dense yellow fog" , "Thick smokey fog" , "Thick, dirty, yellowish fog" .)