More severe misery to involve vergers in Reims (5)
I believe the answer is:
worse
'more severe' is the definition.
'misery to involve vergers in reims' is the wordplay.
'misery' becomes 'woe' (woe is a kind of misery).
'to involve' is an insertion indicator.
'vergers in' means to remove the middle letters (the letters on the edges or 'verges' are used).
'reims' with its middle removed is 'rs'.
'woe' enclosing 'rs' is 'WORSE'.
(Other definitions for worse that I've seen before include "Less good" , "Bad to a greater degree" , "Opposite of better" , "Less favourable" , "Of poorer quality" .)