Rather hidden article (3)
I believe the answer is:
the
'article' is the definition.
(the 'definite article' part of speech)
'rather hidden' is the wordplay.
'hidden' indicates the answer is hidden within the clue.
'THE' is hidden within 'rather'.
(Other definitions for the that I've seen before include "Article is quite definite" , "Over - - moon" , "See 10 Across" , "Born - - - - - - - - (carol)" , "Finis" .)