Part of Israel destined for the firstborn (6)
I believe the answer is:
eldest
'the firstborn' is the definition.
('eldest' can be a synonym of 'firstborn')
'part of israel destined for' is the wordplay.
'part of' indicates a hidden word.
'ELDEST' can be found hidden inside 'israel destined for'.
(Other definitions for eldest that I've seen before include "heir?" , "Firstborn; led set (anag.)" , "Most advanced in years" , "Most old" , "Of greatest age" .)