Dietary necessity for one in ten (7)
I believe the answer is:
protein
'dietary necessity' is the definition.
(I've seen this in another clue)
'for one in ten' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'i' going within 'ten' is 'tein'.
'pro'+'tein'='PROTEIN'
(Other definitions for protein that I've seen before include "Body-builder" , "A body chemical; pointer (anag.)" , "complex compound" , "Substance in meat" , "Not ripe (anag.)" .)