Article subsequently shortened (3)
I believe the answer is:
the
'article' is the definition.
(the 'definite article' part of speech)
'subsequently shortened' is the wordplay.
'subsequently' becomes 'then' ('then' can be a synonym of 'subsequently').
'shortened' means to remove the last letter.
'then' with its last letter taken off is 'THE'.
(Other definitions for the that I've seen before include "and 24 Down: Similar things" , "Born - - - - - - - - (carol)" , "SECOND PART OF 23" , "Article is quite definite" , "The most frequently occurring word in the Bible (and in this clue!)" .)