It's nourishing, with or without a little mustard (9)
I believe the answer is:
nutriment
'mustard' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nutriment that I've seen before include "It sustains" , "diet" , "Nourishing food" , "Material providing food" , "Sustenance, nourishment" .)