Some tartare sauce for Jacob's brother (4)
I believe the answer is:
esau
'jacob's brother?' is the definition.
(I've seen this in another clue)
'some tartare sauce for' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ESAU' can be found hidden inside 'tartare sauce for'.
(Other definitions for esau that I've seen before include "Biblical twin to Jacob" , "Jacob's brother (Gen.)" , "He traded his birthright for a mess of pottage" , "Old Testament twin" , "man tricked by kid brother" .)