Something used for shortening (3)
I believe the answer is:
etc
'something used for shortening' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for etc that I've seen before include "And so forth (abbr)" , "And so on, briefly" , "'And the rest, in short (3)'" , "Etcetera (abbrev.)" , "And so on, in brief" .)