Writer lost out on plaything (7)
I believe the answer is:
tolstoy
'writer' is the definition.
(I've seen this before)
'lost out on plaything' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'on' means one lot of letters goes inside another.
'plaything' becomes 'toy' (synonyms).
'lost' with letters rearranged gives 'olst'.
'olst' placed into 'toy' is 'TOLSTOY'.
(Other definitions for tolstoy that I've seen before include "Author of Anna Karenina, d.1910" , "He wrote Anna Karenina" , "Count Leo . . . . . . ., author of 'War and Peace'" , "Lost toy belonging to Count Leo, writer" , "Writer of War and Peace, d.1910" .)