Writer’s counterpart receiving top mark (5)
I believe the answer is:
twain
'writer's' is the definition.
(author Mark Twain)
'counterpart receiving top mark' is the wordplay.
'counterpart' becomes 'twin' (synonyms).
'receiving' indicates putting letters inside.
'top mark' becomes 'a' (top academic grade).
'twin' going around 'a' is 'TWAIN'.
(Other definitions for twain that I've seen before include "Two, formerly" , "Mark ___ , American writer" , "two in history" , "Mark, US writer" , "Kipling denied ever meeting" .)