Component in cereal is source of protein (5,4)
I believe the answer is:
white meat
'source of protein' is the definition.
'component in cereal' is the wordplay.
'component' becomes 'item' (I've seen this before).
'in' means one lot of letters goes inside another.
'cereal' becomes 'wheat' (wheat is a kind of cereal**).
'item' put inside 'wheat' is 'WHITE MEAT'.
'is' is the link.
(Other definitions for white meat that I've seen before include "chicken perhaps" , "chicken, say" , "Turkey breast, perhaps" , "light meal?" .)