Mark as adult visiting his very closest relative (5)
I believe the answer is:
twain
This clue could be a double definition.
'mark' is the first definition.
(Mark Twain)
'adult visiting his very closest relative' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'as' is the link.
Can you help me to learn more?
(Other definitions for twain that I've seen before include "Mark ..... created Tom Sawyer" , "that was a low number" , "brace" , "Two, formerly" , "Pen name of Samuel Langhorne Clemens, d. 1910)" .)