She writes a letter in French (5)
I believe the answer is:
ellen
'she' is the definition.
'a letter in french' is the wordplay.
'a letter' becomes 'ell' (ell is a kind of letter).
'in french' becomes 'en' ('en' can be French for 'in' e.g. en France).
'ell'+'en'='ELLEN'
'writes' acts as a link.
(Other definitions for ellen that I've seen before include "MacArthur, round-the-world yachtswoman" , "Dame - MacArthur, round-the-world sailor" , "Woman" , "Terry perhaps" , "- Terry, Eng. actress" .)