In parenthesis, believe it or not! (3)
I believe the answer is:
sic
'in' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I don't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sic that I've seen before include "Like" , "poorly spelt if requiring this" , "Thus among musicians" , "Printing term to denote accuracy though it appears otherwise" , "Quoted exactly as in the original; so" .)