Articles about source of tenacious Greek character (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'articles about source of tenacious' is the wordplay.
'articles' becomes 'the a' (two grammatical articles).
'about' is an insertion indicator.
'source of' suggests taking the first letters.
The initial letter of 'tenacious' is 't'.
'thea' going around 't' is 'THETA'.
(Other definitions for theta that I've seen before include "letter from abroad" , "Letter from Greece" , "character in Marathon" , "Foreign letter" , "character from Greece" .)