At midnight everything is bile (4)
I believe the answer is:
gall
'bile' is the definition.
(bile from the gall bladder)
'at midnight everything' is the wordplay.
'at' means one lot of letters go next to another.
'midnight' becomes 'g' (the middle letter in 'night').
'everything' becomes 'all' (all things).
'g'+'all' is 'GALL'.
'is' acts as a link.
(Other definitions for gall that I've seen before include "Impudence; bitterness" , "Impudence - annoy" , "A sore spot or a bodily bladder" , "Bold impudent behaviour; abnormal growth on tree" , "Sheer impudence from bladder?" .)