Most common English word (3)
I believe the answer is:
the
'most common english word' 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 "SECOND PART OF 23" , "See 10 Across" , "Born - - - - - - - - (carol)" , "Over - - moon" , "French drink" .)