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 "Written exactly thus", "Quoted exactly as in the original; so", "So", "Thus in Corsica", "Intentionally so written", "Urge to attack", "As it is in the original", "It calls attention to a quoted mistake", "Thus, sounds ill" and "Word to call attention to a quoted mistake".)