Not this (4)
I believe the answer is:
that
'not this' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for that that I've seen before include "Just like --; -- is to say" , "Take ..., English pop band" , "Yon" , "See 5" , "Not 8 Ac but the other" .)