Thick? Way beyond stupid, on reflection! (5)
I believe the answer is:
'thick?' is the definition.
(I know that midst can be written as midst)
'way beyond stupid on reflection' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'beyond' means one lot of letters go next to another.
'stupid' becomes 'dim' (similar in meaning).
'on reflection' says the letters should be written backwards.
'dim' written backwards gives 'mid'.
'st' put after 'mid' is 'MIDST'.
(Other definitions for midst that I've seen before include "Among, amid" , "Central part" , "Blue dye; rainbow hue" , "Where one is surrounded by other things" , "Thick (of things)" .)