Dietary essential (7)
I believe the answer is:
protein
'dietary essential' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for protein that I've seen before include "Important component of diet" , "Important dietary component" , "Dietary requirement" , "Not ripe (anag.)" , "Dietary component -- 23 across (anag)" .)