Thick fog seen around break of day (5)
I believe the answer is:
midst
'thick' is the definition.
('midst' can be a synonym of 'thick')
'fog seen around break of day' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'seen around' means one lot of letters goes inside another.
'break of day' becomes 'd' (I've seen this in other clues).
'mist' going around 'd' is 'MIDST'.
(Other definitions for midst that I've seen before include "Among, amid" , "Thick (of things)" , "Centre" , "Where one is surrounded by other things" , "Central part" .)