Thus sounds ill (3)
I believe the answer is:
sic
'thus' is the definition.
('thus' in Latin)
'sounds ill' is the wordplay.
'sounds' shows a homophone (sound like).
'ill' becomes 'sick' (synonyms).
'sick' sounds like '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" .)