Spot sibling including a writer (4,5)
I believe the answer is:
mark twain
'writer' is the definition.
(I know this)
'spot sibling including a' is the wordplay.
'spot' becomes 'mark' (I've seen this in other clues).
'sibling' becomes 'twin' (twin is a kind of sibling).
'including' is an insertion indicator.
'twin' enclosing 'a' is 'twain'.
'mark'+'twain'='MARK TWAIN'
(Other definitions for mark twain that I've seen before include "Writer of Huckleberry Finn" , "American humorous writer, d. 1910" , "Tom had him writing about?" , ""The Adventures of Huckleberry Finn" author" , "Creator of Tom Sawyer and Huckleberry Finn" .)