Crossword Genius

Cruel destiny imprisoning the first-born (6)

Ross

I believe the answer is:

eldest

'the first-born' is the definition.
(the eldest child)

'cruel destiny imprisoning' is the wordplay.
'imprisoning' indicates a hidden word (the answer is imprisoned within the clue).
'ELDEST' is hidden in the letters of 'cruel destiny'.

(Other definitions for eldest that I've seen before include "heir?" , "Most senior (E, not O)" , "Most senior of a group" , "Most senior of two" , "Of greatest age" .)

I've seen this clue in The Telegraph.
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