Not ripe perhaps but necessary for one's diets (7)
I believe the answer is:
protein
'necessary for one's diets' is the definition.
I can't tell whether this definition defines the answer.
'not ripe perhaps' is the wordplay.
'perhaps' indicates an anagram.
'not'+'ripe'='notripe'
'notripe' is an anagram of 'PROTEIN'.
'but' is the link.
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Body-builder" , "Combination of amino acids in living things" , "Dietary component -- 23 across (anag)" , "Important component of diet" , "Organic compound, essential constituent of living cells" .)