Thick fog seen around break of day (5)
I believe the answer is:
Here is my best explanation:
'thick' is the definition.
('midst' can be a synonym of 'thick')
'fog seen around break of day' is the subsidiary indication.
'fog' becomes 'mist' (mist is a kind of fog).
'seen around' indicates putting letters inside.
'break of day' becomes 'd' (I've seen this in other clues).
'mist' placed around 'd' is 'midst'.
(Other definitions for midst that I've seen before include "Among, amid", "Thick", "Centre", "Blue dye; rainbow hue", "Central part", "In the middle of (archaic or literary)" and "Where one is surrounded by other things".)