Sounding so ill … (4)
I believe the answer is:
sick
'ill' is the definition.
('sick' can be similar in meaning to 'ill')
'sounding so' is the wordplay.
'sounding' indicates a 'sounds like' (homophone) clue.
'so' becomes 'sic' ('sic' can be a synonym of 'so').
'sic' sounds like 'SICK'.
(Other definitions for sick that I've seen before include "Bad" , "Ailing" , "Ill; nauseous" , "well? Hardly" , "Poorly" .)