So is not well spoken (3)
I believe the answer is:
sic
'so' is the definition.
('so' in Latin)
'not well spoken' is the wordplay.
'not well' becomes 'sick' ('sick' is the opposite to 'well').
'spoken' shows a homophone (sound like).
'sick' is a homophone of 'SIC'.
'is' acts as a link.
(Other definitions for sic that I've seen before include "Quoted exactly as in the original; so" , "Thus in Corsica" , "poorly spelt if requiring this" , "Word to call attention to a quoted mistake" , "Thus, sounds ill" .)