Definitely an article (3)
I believe the answer is:
the
'article' is the definition.
(the definite article in English)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for the that I've seen before include "and 24 Down: Similar things" , "Born - - - - - - - - (carol)" , "The most frequently occurring word in the Bible (and in this clue!)" , "Finis" , "Base of ever-expanding nursery rhyme" .)