Sarah's son (5)
I believe the answer is:
isaac
'sarah's son' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for isaac that I've seen before include "Actor Oscar" , "Father to Jacob and Esau" , "biblical figure" , "-- Asimov, science fiction writer" , "Son of Abraham, offered as a sacrifice to God" .)