Articles about the leader’s letter (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter)
'articles about the leader's' is the wordplay.
'articles' becomes 'thea' (two examples of the part-of-speech: the+a).
'about' is an insertion indicator.
'leader' indicates taking the first letters.
The first letter of 'the' is 't'.
'thea' enclosing 't' is 'THETA'.
(Other definitions for theta that I've seen before include "Make amends for" , "Letter to the Corinthians" , "letter from abroad" , "Greek character" , "Eighth letter of Greek alphabet" .)