Nourishing substance (4)
(Other definitions for food that I've seen before include "Solid nourishment" , "Sustenance" , "Edibles" , "Nutriment" , "Eatables" .)
'nourishing substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nutrient that I've seen before include "Source of nourishment" , "Substance providing nourishment" , "Substance to give energy and build tissue" , "Feeding" , "Substance essential for growth" .)