An article in leather (3)
I believe the answer is:
the
'an article' is the definition.
(the 'definite article' part of speech)
'in leather' is the wordplay.
'in' indicates a hidden word.
'THE' is hidden in the letters of 'leather'.
(Other definitions for the that I've seen before include "See 10 Across" , "Last words of story" , "That particular one" , "Finis" , "French drink" .)