Thus, to Tacitus (3)
I believe the answer is:
sic
'thus to tacitus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sic that I've seen before include "Quoted exactly as in the original; so" , "Urge to attack" , "'Thus, in printing (3)'" , "Thus, sounds ill" , "poorly spelt if requiring this" .)