Sibling, for short (3)
I believe the answer is:
sis
'sibling for short' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sis that I've seen before include "Girl" , "Secret Intelligence Service (abbreviation)" , "Relative's" , "Secret Intelligence Service (abbr.)" , "Sibling (informal)" .)