Isaac's firstborn (4)
I believe the answer is:
esau
'isaac's firstborn' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for esau that I've seen before include "Jacob's brother (OT)" , "wronged brother" , "Big Brother" , "Biblical man who sold his birthright for a mess of pottage" , "Elder of the twin sons of Isaac and Rebecca" .)