Thus sounds ill (3)
I believe the answer is:
Here is my best explanation:
'thus' is the definition.
('sic' can be a synonym of 'thus')
'sounds ill' is the subsidiary indication.
'sounds' shows a homophone (sound like).
'ill' becomes 'sick' ('sick' can be a synonym of 'ill').
'sick' sounds like 'sic'.
(Other definitions for sic that I've seen before include "Printing term to denote accuracy though it appears otherwise", "Like", "Thus in classical way", "poorly spelt if requiring this", "Exactly as written", "Urge to attack", "Intentionally so written", "Thus in Corsica", "Quoted exactly as in the original; so" and "(Of quoted word) exactly so".)