US author may finally retire (9)
I believe the answer is:
hawthorne
'us author' is the definition.
(Nathaniel Hawthorne)
'may finally retire' is the wordplay.
'may' becomes 'hawthorn' (may is another name for the hawthorn tree).
'finally' says to take the final letters.
The last letter of 'retire' is 'e'.
'hawthorn'+'e'='HAWTHORNE'
(Other definitions for hawthorne that I've seen before include "Novelist" , "Nathaniel ......... wrote ` `The Scarlet Letter' '" , "Nathaniel . . . . . . . . ., American author of 'The Scarlet Letter" , "One of the US authors" .)