Definite article (3)
I believe the answer is:
the
'definite article' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for the that I've seen before include "and 24 Down: Similar things" , "Over - - moon" , "That particular one" , "Article is quite definite" , "See 10 Across" .)