Thus sounds ill (3)
I believe the answer is:
sic
'thus' is the definition.
('sic' can be a synonym of 'thus')
'sounds ill' is the wordplay.
'sounds' shows a homophone (sound like).
'ill' becomes 'sick' ('sick' can be a synonym of 'ill').
'sick' is a homophone of 'SIC'.
(Other definitions for sic that I've seen before include "Printing term to denote accuracy though it appears otherwise" , "Thus in classical way" , "poorly spelt if requiring this" , "Thus in Corsica" , "Intentionally so written" .)