Isaac's eldest (4)
I believe the answer is:
esau
'isaac's eldest' 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 "Hairy OT character" , "Son of Isaac, brother to Jacob (Bible)" , "Rebecca's son" , "brother done in Bible" , "Twin brother of Jacob" .)