An article every fathead needs (3)
I believe the answer is:
the
'an article' is the definition.
(the definite article in English)
'every fathead needs' is the wordplay.
'needs' indicates the answer is hidden within the clue.
'THE' can be found hidden inside 'every fathead'.
(Other definitions for the that I've seen before include "Born - - - - - - - - (carol)" , "Base of ever-expanding nursery rhyme" , "Over - - moon" , "That particular one" , "SECOND PART OF 23" .)