Mustard relative (5)
I believe the answer is:
cress
'mustard relative' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cress that I've seen before include "Salad plant paired with mustard" , "Toll of bell" , "Green cold salad ingredient" , "Salad sprouts" , "bit of salad" .)