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