Niacin or riboflavin, e.g. (7)

I believe the answer is:

vitamin

(Other definitions for vitamin that I've seen before include "Organic substance essential to life" , "Essential diet compound" , "Riboflavin, for example" , "Food constituent" , "A, B, C, D, E or K" .)

I've seen this clue in The Wall Street Journal.

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