Unhappy eating starter of nutritious grains (4)
I believe the answer is:
'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 "Result of rock erosion", "Shore grain", "Very fine grains of rock, as on beach", "lover of Chopin", "Content of hourglass".)