Just one of those things (4)
I believe the answer is:
that
'just one of those things' 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 "Take ..., English pop band" , "Not 8 Ac but the other" , "The (one) there" , "Yon" , "See 5" .)