Unlawful (12)
There are a few possibilities for this:
illegitimate
'unlawful' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for illegal that I've seen before include "Black" , "Not in conformity with law" , "Felonious" , "Unlawful" , "Forbidden" .)
'unlawful' is the definition.
(I know that unlawful can be written as illicit)
This is all the clue.
(Other definitions for illicit that I've seen before include "wrong" , "Not lawful" , "Not legal" , "completely banned" , "Unacceptable" .)
'unlawful' is the definition.
(synonyms)
This is all the clue.
(Other definitions for illegitimate that I've seen before include "Not in accordance with the law" , "Not proper, acceptable" , "Unlawful, invalid" , "Not allowed" , "Improper" .)