She writes an article on exotic trees (6)
I believe the answer is:
teresa
'she' is the definition.
'an article on exotic trees' is the wordplay.
'an article' becomes 'a' (the indefinite article in English).
'on' means one lot of letters go next to another.
'exotic' indicates an anagram (an unusual form of the letters).
'trees' with letters rearranged gives 'teres'.
'a' after 'teres' is 'TERESA'.
'writes' is the link.
(Other definitions for teresa that I've seen before include "Saint" , "Woman" , "Mother...saintly nun of Calcutta" , "Does 'e stare at the girl?" , "Mother ..., who ministered to the poor in India" .)