Article they shortened (3)
I believe the answer is:
the
'article' is the definition.
(the 'definite article' part of speech)
'they shortened' is the wordplay.
'shortened' means to remove the last letter.
'they' with its last letter taken off is 'THE'.
(Other definitions for the that I've seen before include "15, 16 One given taste of own medicine" , "That particular one" , "French drink" , "Over - - moon" , "The most frequently occurring word in the Bible (and in this clue!)" .)