Remember - two Finns were involved in this guy's plots (4,5)
I believe the answer is:
mark twain
'finns were involved in this guy's plots' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'remember two' is the wordplay.
'remember' becomes 'mark' (I can't explain this - if you can you should believe this answer much more).
'two' becomes 'twain' (archaic term for 'two').
'mark'+'twain'='MARK TWAIN'
Can you help me to learn more?
(Other definitions for mark twain that I've seen before include ""The Adventures of Huckleberry Finn" author" , "Tom had him writing about?" , "American humorous writer, d. 1910" , "Creator of Tom Sawyer and Huckleberry Finn" , "Writer of Huckleberry Finn" .)