In port, perhaps, an American writer (6)
I believe the answer is:
proust
'writer' is the definition.
(I know that Proust is a novelist)
'in port perhaps an american' is the wordplay.
'in' means one lot of letters goes inside another.
'perhaps' indicates an anagram.
'an american' becomes 'US' (abbreviation for United States).
'port' with letters rearranged gives 'prot'.
'prot' enclosing 'us' is 'PROUST'.
(Other definitions for proust that I've seen before include "Remembrance of Things Past author" , "Great French writer in stupor" , "See 8" , "Marcel ---, Fr. author" , "French author" .)