Article thanks Greek character (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'article thanks' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the'+'ta'='THETA'
(Other definitions for theta that I've seen before include "letter from Athens" , "Letter to the Corinthians" , "letter to Greeks" , "Letter before iota" , "Foreign character" .)