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