Unhappy eating starter of nutritious grains (4)
I believe the answer is:
sand
'grains' is the definition.
(I've seen this before)
'unhappy eating starter of nutritious' is the wordplay.
'unhappy' becomes 'sad' (synonyms).
'eating' means one lot of letters goes inside another.
'starter of' says to take the initial letters.
The first letter of 'nutritious' is 'n'.
'sad' placed around 'n' is 'SAND'.
(Other definitions for sand that I've seen before include "Fine stone particles" , "Loose grains" , "Egg timer grains" , "Desert grit" , "Seaside castle-building material" .)