Article shared by heathen, atheist and others (3)
I believe the answer is:
the
'article' is the definition.
(the 'definite article' part of speech)
'shared by heathen atheist and others' is the wordplay.
'THE' is hidden within the letters.
However, I am unsure how the hidden word is indicated.
(Other definitions for the that I've seen before include "Born - - - - - - - - (carol)" , "Last words of story" , "Base of ever-expanding nursery rhyme" , "See 10 Across" , "15, 16 One given taste of own medicine" .)