Miserable day in which to be sick (8)
I believe the answer is:
wretched
'miserable' is the definition.
('wretched' can be a synonym of 'miserable')
'day in which to be sick' is the wordplay.
'day' becomes 'wed' (abbreviation for Wednesday).
'in which' indicates putting letters inside.
'to be sick' becomes 'retch' ('retch' can be a synonym of 'be sick').
'wed' going around 'retch' is 'WRETCHED'.
(Other definitions for wretched that I've seen before include "Most unfortunate, piteous" , "Sorry!" , "Very unhappy, miserable" , "Distressingly bad" , "Abject" .)