Source of protein
I believe the answer is:
meat
'source of protein' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for meat that I've seen before include "Beef, chicken, pork etc" , "Animal flesh as food" , "Ham, pork, chicken etc" , "Chops, steaks etc" , "Spam, ham, lamb etc" .)