Amino acid chain (7)
I believe the answer is:
protein
'amino acid chain' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for protein that I've seen before include "Organic compound, essential constituent of living cells" , "Key part of healthy diet" , "Substance in meat" , "Not ripe (anag.)" , "Essential ingredient of food" .)