Firstborn in, Rachel, destined to be here (6)
I believe the answer is:
eldest
'firstborn' is the definition.
(synonyms)
'rachel destined to be here' is the wordplay.
'ELDEST' is hidden in amongst the letters.
But, I'm not sure how the hidden word is indicated.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for eldest that I've seen before include "Most aged" , "First in the order of birth" , "Such as Cain, say" , "Most senior of a group" , "Of greatest age" .)