Caesar's so poorly, we hear (3)
I believe the answer is:
sic
'caesar's so' is the definition.
('so' in Latin)
'poorly we hear' is the wordplay.
'poorly' becomes 'sick' (similar in meaning).
'we hear' shows a homophone (sound like).
'sick' is a homophone of 'SIC'.
(Other definitions for sic that I've seen before include "Word to call attention to a quoted mistake" , "Thus - in Latin" , "Written exactly thus" , "Quoted exactly as in the original; so" , "(Of quoted word) exactly so" .)