Is round about? Yes, really (3)
I believe the answer is:
Here is my best explanation:
'yes really' is the definition.
(used to show a quoted error is not itself an error)
'is round about?' is the subsidiary indication.
'round' is a reversal indicator.
'about?' becomes 'c' (abbreviation for 'circa').
'is' reversed gives 'si'.
(Other definitions for sic that I've seen before include "(Of quoted word) exactly so", "Thus in Corsica", "Quoted exactly as in the original; so", "Like", "Written exactly thus", "So", "poorly spelt if requiring this", "Urge to attack", "Thus - in Latin" and "It calls attention to a quoted mistake".)