Doppelganger protecting a writer (5)
I believe the answer is:
twain
'writer' is the definition.
(author Mark Twain)
'doppelganger protecting a' is the wordplay.
'doppelganger' becomes 'twin' (I've seen this in another clue).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'twin' enclosing 'a' is 'TWAIN'.
(Other definitions for twain that I've seen before include "Kipling denied ever meeting" , "Mark, say" , "brace" , "Tom Sawyer author, d. 1910" , "Mark ..... created Huckleberry Finn" .)