Substance found in beans for can, energy packed in (7)
I believe the answer is:
protein
'substance' is the definition.
(protein is a kind of substance)
'for can energy packed in' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'can' becomes 'tin' (both are metal containers).
'energy' becomes 'e' (physics symbol as in E=mc2).
'packed in' is an insertion indicator.
'tin' going around 'e' is 'tein'.
'pro'+'tein'='PROTEIN'
'found in beans' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Combination of amino acids in living matter" , "Body-builder" , "Essential compound of food" , "Dietary component -- 23 across (anag)" , "Combination of amino acids in living things" .)