Less well (5)
'less well' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for worse that I've seen before include "More awful or unwell" , "Bad to a greater degree" , "Opposite of better" , "Of poorer quality" , "Less satisfactory" .)
'less well' is the definition.
('ill' can be the opposite of 'well' so perhaps 'iller' is reasonable for 'less well')
This is the entire clue.
(Other definitions for iller that I've seen before include "Even more 9" , "In worse health" , "Less well" .)