For one in ten, diet needs this (7)
I believe the answer is:
protein
'diet needs this' is the definition.
The definition suggests a singular noun which matches the answer.
'for one in ten' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'i' going into 'ten' is 'tein'.
'pro'+'tein'='PROTEIN'
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Dietary requirement" , "Important dietary component" , "Essential ingredient of food" , "Essential compound of food" , "Combination of amino acids in living matter" .)