Thick fog that envelops delta (5)
I believe the answer is:
midst
'thick' is the definition.
(synonyms)
'fog that envelops delta' is the wordplay.
'fog' becomes 'mist' (synonyms).
'that envelops' means one lot of letters goes inside another (I've seen this in other clues).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'mist' enclosing 'd' is 'MIDST'.
(Other definitions for midst that I've seen before include "Central part" , "Where one is surrounded by other things" , "Centre" , "Among, amid" , "In the middle of (archaic or literary)" .)