Letter of thanks following article (5)
I believe the answer is:
theta
'letter' is the definition.
(Greek letter)
'thanks following article' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'following' says to put letters next to each other.
'article' becomes 'the' (the definite article in English).
'ta' put after 'the' is 'THETA'.
'of' is the link.
(Other definitions for theta that I've seen before include "Homeric character" , "classic character?" , "Foreign character" , "character in Marathon" , "letter to Greeks" .)