Two articles about tense letter (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter)
'two articles about tense' is the wordplay.
'two articles' becomes 'thea' (article meaning part of speech: the, a etc.).
'about' means one lot of letters goes inside another.
'tense' becomes 't' (abbreviation used in many dictionaries).
'thea' placed around 't' is 'THETA'.
(Other definitions for theta that I've seen before include "Letter from Greece" , "Letter before iota" , "Make amends for" , "Foreign character" , "8th letter of the Greek alphabet" .)