Article thanks foreign character (5)
I believe the answer is:
theta
'foreign character' is the definition.
(Greek letter)
'article thanks' is the wordplay.
'article' becomes 'the' (the definite article in English).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the'+'ta'='THETA'
(Other definitions for theta that I've seen before include "Eighth letter of Greek alphabet" , "Foreign letter" , "character in Marathon" , "Letter from Greece" , "Greek letter equivalent to th" .)