Article gets thank you letter (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter of the alphabet)
'article gets thank you' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'gets' says to put letters next to each other.
'thank you' becomes 'ta'.
'the'+'ta'='THETA'
(Other definitions for theta that I've seen before include "Make amends for" , "classic character?" , "letter from Athens" , "Foreign letter" , "Letter from Greece" .)