Some white sauce for Jacob's brother (4)
I believe the answer is:
esau
'jacob's brother' is the definition.
(I've seen this before)
'some white sauce for' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ESAU' is hidden in the letters of 'white sauce for'.
(Other definitions for esau that I've seen before include "Twin brother of Jacob" , "man tricked by kid brother" , "First son of Isaac (Bible)" , "Biblical man who sold his birthright for a mess of pottage" , "Isaac's eldest son" .)