Essential part of diet is not ripe, unfortunately (7)
I believe the answer is:
protein
'essential part of diet' is the definition.
'not ripe unfortunately' is the wordplay.
'unfortunately' indicates an anagram.
'not'+'ripe'='notripe'
'notripe' anagrammed gives 'PROTEIN'.
'is' is the link.
(Other definitions for protein that I've seen before include "Essential part of diet, found in meat, say" , "Important component of diet" , "Substance in meat" , "complex compound" , "Important dietary component" .)