Biblical patriarch, the son of Abraham (5)
I believe the answer is:
isaac
I believe this clue is a double definition.
'biblical patriarch' is the first definition.
(I've seen this in another clue)
'the son of abraham' is the second definition.
(I've seen this before)
(Other definitions for isaac that I've seen before include "Sarah's son" , "Father to Jacob and Esau" , "- Newton, Eng. physicist and mathematician" , "Father of 21" , "- Asimov, sci-fi writer" .)