Sauce like this always served up (4)
I believe the answer is:
soya
'sauce' is the definition.
(I have seen 'Dark sauce ' mean 'soya' so perhaps 'sauce' could also mean 'soya')
'like this always served up' is the wordplay.
'like this' becomes 'so' ('like so').
'always' becomes 'ay' (archaic term for 'always').
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'ay' reversed gives 'ya'.
'so'+'ya'='SOYA'
(Other definitions for soya that I've seen before include "I'm rich in oil" , "Plant grown for its beans" , "Protein-rich vegetable" , "Oily bean" , "It's used as a substitute for animal protein" .)