Nutritionally, something for youth, it's said (7)
I believe the answer is:
protein
'nutritionally something' is the definition.
I can't judge whether this definition defines the answer.
'for youth it's said' is the wordplay.
I cannot quite see how this works, but
'for' could be 'pro' (I've seen this before) and 'pro' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'ein' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Important component of diet" , "Combination of amino acids in living matter" , "Essential ingredient of food" , "Substance in meat" , "A body chemical; pointer (anag.)" .)