Some high-level destiny for first-born (6)
I believe the answer is:
'for first-born' is the definition.
(the eldest child)
'some high-level destiny' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ELDEST' is hidden within 'high level destiny'.
(Other definitions for eldest that I've seen before include "most early in origin" , "Most senior (E, not O)" , "Firstborn; led set (anag.)" , "Most aged" , "The oldest of three or more" .)