In favour of youngsters reportedly getting dietary requirements (8)
I believe the answer is:
proteins
'dietary requirements' is the definition.
(proteins are essential parts of one's diet)
'in favour of youngsters reportedly getting' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'youngsters' becomes 'teens' (I've seen this in another clue).
'reportedly' indicates a 'sounds like' (homophone) clue.
'getting' says to put letters next to each other.
'teens' is a homophone of 'teins'.
'pro'+'teins'='PROTEINS'
(Other definitions for proteins that I've seen before include "Pointers to organic compounds containing nitrogen, essential in food" , "Organic compounds, obtained from meat, eggs, etc." , "Long strings of amino acids" .)