In parenthesis, an assurance of correctness (3)
I believe the answer is:
sic
'in parenthesis an assurance of correctness' is the definition.
I can't tell whether this definition defines the answer.
'in parenthesis an' is the wordplay.
'in' means one lot of letters goes inside another.
'parenthesis' becomes 'sc' (I can't justify this - if you can you should give a lot more credence to this answer).
'an' becomes 'i' (Roman numeral for one).
'sc' placed around 'i' is 'SIC'.
Can you help me to learn more?
(Other definitions for sic that I've seen before include "the reading is correct" , "Intentionally so written" , "As it stands, in printing" , "Thus, sounds ill" , "Like" .)