Letter or article accompanying grateful comment (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter)
'article accompanying grateful comment' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'accompanying' says to put letters next to each other.
'grateful comment' becomes 'ta' (ta can mean thank you).
'the'+'ta'='THETA'
'or' acts as a link.
(Other definitions for theta that I've seen before include "Letter to the Corinthians" , "letter from Athens" , "Foreign letter" , "8th letter of the Greek alphabet" , "Make amends for" .)