Supporting number to accommodate one part of diet (7)
I believe the answer is:
protein
'diet' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'supporting number to accommodate one part' is the wordplay.
'supporting' becomes 'pro' (being pro something is supporting it).
'number' becomes 'ten' (ten is a number).
'to accommodate one' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'part' means one lot of letters goes inside another (the inserted letters split open or part the word).
'ten' enclosing 'i' is 'tein'.
'pro'+'tein'='PROTEIN'
'of' is the link.
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Important dietary component" , "Essential part of diet, found in meat, say" , "Important part of diet" , "Dietary component -- 23 across (anag)" , "Combination of amino acids in living matter" .)