Source of protein in mayo served up (4)
I believe the answer is:
soya
'source of protein' is the definition.
(I've seen this before)
'in mayo served up' is the wordplay.
'in' indicates a hidden word.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ayos' can be found hidden inside 'mayo served'.
'ayos' in reverse letter order is 'SOYA'.
(Other definitions for soya that I've seen before include "Vegetable with very high protein" , "Protein-rich vegetable" , "I'm rich in oil" , "Tofu source" , "Form of protein derived from beans" .)