Most common word in English (3)
I believe the answer is:
the
'most common word in english' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for the that I've seen before include "SECOND PART OF 23" , "Over - - moon" , "Born - - - - - - - - (carol)" , "and 24 Down: Similar things" , "That particular one" .)