Bird with some backward possibilities (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'some backward possibilities' is the wordplay.
'backward' says the letters should be written in reverse.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'sibi' is hidden within 'possibilities'.
'sibi' written backwards gives 'IBIS'.
'with' acts as a link.
(Other definitions for ibis that I've seen before include "may be sacred?" , "Wader once sacred in Egypt" , "Wading bird with a down-curved bill" , "Storklike bird" , "Long-legged wading bird" .)