Some cruel destiny for first-born (6)
I believe the answer is:
eldest
'first-born' is the definition.
(the eldest child)
'some cruel destiny for' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ELDEST' is hidden in the letters of 'cruel destiny for'.
(Other definitions for eldest that I've seen before include "Most senior in age" , "Most aged" , "Firstborn; led set (anag.)" , "Most advanced in years" , "Most senior of a group" .)