Taste set dinner with mustard and cress, primarily, in a salad (11)
I believe the answer is:
discernment
'taste' is the definition.
(synonyms)
I don't understand the rest of the clue.
(Other definitions for discernment that I've seen before include "Clear perception" , "Understanding" , "good judgement's needed here" , "Judgement; acuteness" , "Good judgement" .)