Greek character and article coming with thanks (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'article coming with thanks' is the wordplay.
'article' becomes 'the' (the definite article in English).
'coming with' means one lot of letters go next to another.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the'+'ta'='THETA'
'and' acts as a link.
(Other definitions for theta that I've seen before include "letter from Argos?" , "letter to Greeks" , "Eighth letter of Greek alphabet" , "Make amends for" , "character in Marathon" .)